Parsers.scala

package dotty.tools
package dotc
package parsing

import scala.language.unsafeNulls

import scala.annotation.internal.sharable
import scala.collection.mutable.ListBuffer
import scala.collection.immutable.BitSet
import util.{ SourceFile, SourcePosition, NoSourcePosition }
import Tokens.*
import Scanners.*
import xml.MarkupParsers.MarkupParser
import core.*
import Flags.*
import Contexts.*
import Names.*
import NameKinds.{WildcardParamName, QualifiedName}
import NameOps.*
import ast.{Positioned, Trees}
import ast.Trees.*
import StdNames.*
import util.Spans.*
import Constants.*
import Symbols.NoSymbol
import ScriptParsers.*
import Decorators.*
import util.Chars
import scala.annotation.tailrec
import rewrites.Rewrites.{patch, overlapsPatch}
import reporting.*
import config.Feature
import config.Feature.{sourceVersion, migrateTo3, globalOnlyImports}
import config.SourceVersion.*
import config.SourceVersion

object Parsers {

  import ast.untpd.*

  case class OpInfo(operand: Tree, operator: Ident, offset: Offset)

  enum Location(val inParens: Boolean, val inPattern: Boolean, val inArgs: Boolean):
    case InParens      extends Location(true, false, false)
    case InArgs        extends Location(true, false, true)
    case InColonArg    extends Location(false, false, true)
    case InPattern     extends Location(false, true, false)
    case InGuard       extends Location(false, false, false)
    case InPatternArgs extends Location(false, true, true) // InParens not true, since it might be an alternative
    case InBlock       extends Location(false, false, false)
    case ElseWhere     extends Location(false, false, false)

  enum ParamOwner:
    case Class           // class or trait or enum
    case CaseClass       // case class or enum case
    case Type            // type alias or abstract type
    case TypeParam       // type parameter
    case Def             // method
    case Given           // given definition
    case ExtensionPrefix // extension clause, up to and including extension parameter
    case ExtensionFollow // extension clause, following extension parameter

    def isClass = // owner is a class
      this == Class || this == CaseClass
    def takesOnlyUsingClauses = // only using clauses allowed for this owner
      this == Given || this == ExtensionFollow
    def acceptsVariance =
      this == Class || this == CaseClass || this == Type

  end ParamOwner

  enum ParseKind:
    case Expr, Type, Pattern

  type StageKind = Int
  object StageKind {
    val None = 0
    val Quoted = 1
    val Spliced = 1 << 1
    val QuotedPattern = 1 << 2
  }

  extension (buf: ListBuffer[Tree])
    def +++=(x: Tree) = x match {
      case x: Thicket => buf ++= x.trees
      case x => buf += x
    }

  /** The parse starting point depends on whether the source file is self-contained:
   *  if not, the AST will be supplemented.
   */
  def parser(source: SourceFile)(using Context): Parser =
    if source.isSelfContained then new ScriptParser(source)
    else new Parser(source)

  private val InCase: Region => Region = Scanners.InCase(_)
  private val InCond: Region => Region = Scanners.InParens(LPAREN, _)
  private val InFor : Region => Region = Scanners.InBraces(_)

  abstract class ParserCommon(val source: SourceFile)(using Context) {

    val in: ScannerCommon

    /* ------------- POSITIONS ------------------------------------------- */

    /** Positions tree.
     *  If `t` does not have a span yet, set its span to the given one.
     */
    def atSpan[T <: Positioned](span: Span)(t: T): T =
      if (t.span.isSourceDerived) t else t.withSpan(span.union(t.span))

    def atSpan[T <: Positioned](start: Offset, point: Offset, end: Offset)(t: T): T =
      atSpan(Span(start, end, point))(t)

    /** If the last read offset is strictly greater than `start`, assign tree
     *  the span from `start` to last read offset, with given point.
     *  If the last offset is less than or equal to start, the tree `t` did not
     *  consume any source for its construction. In this case, don't assign a span yet,
     *  but wait for its span to be determined by `setChildSpans` when the
     *  parent node is positioned.
     */
    def atSpan[T <: Positioned](start: Offset, point: Offset)(t: T): T =
      if (in.lastOffset > start) atSpan(start, point, in.lastOffset)(t) else t

    def atSpan[T <: Positioned](start: Offset)(t: T): T =
      atSpan(start, start)(t)

    def startOffset(t: Positioned): Int =
      if (t.span.exists) t.span.start else in.offset

    def pointOffset(t: Positioned): Int =
      if (t.span.exists) t.span.point else in.offset

    def endOffset(t: Positioned): Int =
      if (t.span.exists) t.span.end else in.lastOffset

    def nameStart: Offset =
      if (in.token == BACKQUOTED_IDENT) in.offset + 1 else in.offset

    /** in.offset, except if this is at a new line, in which case `lastOffset` is preferred. */
    def expectedOffset: Int = {
      val current = in.sourcePos()
      val last = in.sourcePos(in.lastOffset)
      if (current.line != last.line) in.lastOffset else in.offset
    }

    /* ------------- ERROR HANDLING ------------------------------------------- */
    /** The offset where the last syntax error was reported, or if a skip to a
     *  safepoint occurred afterwards, the offset of the safe point.
     */
    protected var lastErrorOffset : Int = -1

    /** Issue an error at given offset if beyond last error offset
     *  and update lastErrorOffset.
     */
    def syntaxError(msg: Message, offset: Int = in.offset): Unit =
      if offset > lastErrorOffset then
        val length = if offset == in.offset && in.name != null then in.name.show.length else 0
        syntaxError(msg, Span(offset, offset + length))
        lastErrorOffset = in.offset

    /** Unconditionally issue an error at given span, without
     *  updating lastErrorOffset.
     */
    def syntaxError(msg: Message, span: Span): Unit =
      report.error(msg, source.atSpan(span))

    def unimplementedExpr(using Context): Select =
      Select(scalaDot(nme.Predef), nme.???)
  }

  trait OutlineParserCommon extends ParserCommon {
    def accept(token: Int): Int

    def skipBracesHook(): Option[Tree]
    def skipBraces(): Unit = {
      accept(if (in.token == INDENT) INDENT else LBRACE)
      var openBraces = 1
      while (in.token != EOF && openBraces > 0)
        skipBracesHook() getOrElse {
          if (in.token == LBRACE || in.token == INDENT) openBraces += 1
          else if (in.token == RBRACE || in.token == OUTDENT) openBraces -= 1
          in.nextToken()
        }
    }
  }

  class Parser(source: SourceFile)(using Context) extends ParserCommon(source) {

    val in: Scanner = new Scanner(source, profile = Profile.current)
    // in.debugTokenStream = true    // uncomment to see the token stream of the standard scanner, but not syntax highlighting

    /** This is the general parse entry point.
     *  Overridden by ScriptParser
     */
    def parse(): Tree = {
      val t = compilationUnit()
      accept(EOF)
      t
    }

/* -------------- TOKEN CLASSES ------------------------------------------- */

    def isIdent = in.isIdent
    def isIdent(name: Name) = in.isIdent(name)
    def isPureArrow(name: Name): Boolean = isIdent(name) && Feature.pureFunsEnabled
    def isPureArrow: Boolean = isPureArrow(nme.PUREARROW) || isPureArrow(nme.PURECTXARROW)
    def isErased = isIdent(nme.erased) && in.erasedEnabled
    // Are we seeing an `erased` soft keyword that will not be an identifier?
    def isErasedKw = isErased && in.isSoftModifierInParamModifierPosition
    def isSimpleLiteral =
      simpleLiteralTokens.contains(in.token)
      || isIdent(nme.raw.MINUS) && numericLitTokens.contains(in.lookahead.token)
    def isLiteral = literalTokens contains in.token
    def isNumericLit = numericLitTokens contains in.token
    def isTemplateIntro = templateIntroTokens contains in.token
    def isDclIntro = dclIntroTokens contains in.token
    def isStatSeqEnd = in.isNestedEnd || in.token == EOF || in.token == RPAREN
    def mustStartStat = mustStartStatTokens contains in.token

    /** Is current token a hard or soft modifier (in modifier position or not)? */
    def isModifier: Boolean = modifierTokens.contains(in.token) || in.isSoftModifier

    def isBindingIntro: Boolean = {
      in.token match {
        case USCORE => true
        case IDENTIFIER | BACKQUOTED_IDENT =>
          in.lookahead.isArrow
        case LPAREN =>
          val lookahead = in.LookaheadScanner()
          lookahead.skipParens()
          lookahead.isArrow
        case _ => false
      }
    } && !in.isSoftModifierInModifierPosition

    def isExprIntro: Boolean =
      in.canStartExprTokens.contains(in.token)
      && !in.isSoftModifierInModifierPosition
      && !(isIdent(nme.extension) && followingIsExtension())

    def isDefIntro(allowedMods: BitSet, excludedSoftModifiers: Set[TermName] = Set.empty): Boolean =
      in.token == AT
      || defIntroTokens.contains(in.token)
      || allowedMods.contains(in.token)
      || in.isSoftModifierInModifierPosition && !excludedSoftModifiers.contains(in.name)

    def isStatSep: Boolean = in.isStatSep

    /** A '$' identifier is treated as a splice if followed by a `{`.
     *  A longer identifier starting with `$` is treated as a splice/id combination
     *  in a quoted block '{...'
     */
    def isSplice: Boolean =
      in.token == IDENTIFIER && in.name(0) == '$' && {
        if in.name.length == 1 then in.lookahead.token == LBRACE
        else (staged & StageKind.Quoted) != 0
      }

/* ------------- ERROR HANDLING ------------------------------------------- */

    /** Is offset1 less or equally indented than offset2?
     *  This is the case if the characters between the preceding end-of-line and offset1
     *  are a prefix of the characters between the preceding end-of-line and offset2.
     */
    def isLeqIndented(offset1: Int, offset2: Int): Boolean = {
      def recur(idx1: Int, idx2: Int): Boolean =
        idx1 == offset1 ||
        idx2 < offset2 && source(idx1) == source(idx2) && recur(idx1 + 1, idx2 + 1)
      recur(source.startOfLine(offset1), source.startOfLine(offset2))
    }

    /** Skip on error to next safe point.
     */
    def skip(): Unit =
      in.skip()
      lastErrorOffset = in.offset

    def warning(msg: Message, offset: Int = in.offset): Unit =
      report.warning(msg, source.atSpan(Span(offset)))

    def deprecationWarning(msg: Message, offset: Int = in.offset): Unit =
      report.deprecationWarning(msg, source.atSpan(Span(offset)))

    /** Issue an error at current offset that input is incomplete */
    def incompleteInputError(msg: Message): Unit =
      if in.offset != lastErrorOffset then
        report.incompleteInputError(msg, source.atSpan(Span(in.offset)))

    /** If at end of file, issue an incompleteInputError.
     *  Otherwise issue a syntax error and skip to next safe point.
     */
    def syntaxErrorOrIncomplete(msg: Message, offset: Int = in.offset): Unit =
      if in.token == EOF then
        incompleteInputError(msg)
      else
        syntaxError(msg, offset)
        skip()

    def syntaxErrorOrIncomplete(msg: Message, span: Span): Unit =
      if in.token == EOF then
        incompleteInputError(msg)
      else
        syntaxError(msg, span)
        skip()

    /** Consume one token of the specified type, or
      * signal an error if it is not there.
      *
      * @return The offset at the start of the token to accept
      */
    def accept(token: Int): Int =
      val offset = in.offset
      if in.token != token then
        syntaxErrorOrIncomplete(ExpectedTokenButFound(token, in.token))
      if in.token == token then in.nextToken()
      offset

    def accept(name: Name): Int = {
      val offset = in.offset
      if !isIdent(name) then
        syntaxErrorOrIncomplete(em"`$name` expected")
      if isIdent(name) then
        in.nextToken()
      offset
    }

    def acceptColon(): Int =
      val offset = in.offset
      if in.isColon then { in.nextToken(); offset }
      else accept(COLONop)

    /** semi = nl {nl} | `;'
     *  nl  = `\n' // where allowed
     */
    def acceptStatSep(): Unit =
      if in.isNewLine then in.nextToken() else accept(SEMI)

    /** Parse statement separators and end markers. Ensure that there is at least
     *  one statement separator unless the next token terminates a statement´sequence.
     *  @param   stats      the statements parsed to far
     *  @param   noPrevStat true if there was no immediately preceding statement parsed
     *  @param   what       a string indicating what kind of statement is parsed
     *  @param   altEnd     a token that is also considered as a terminator of the statement
     *                      sequence (the default `EOF` already assumes to terminate a statement
     *                      sequence).
     *  @return  true if the statement sequence continues, false if it terminates.
     */
    def statSepOrEnd[T <: Tree](stats: ListBuffer[T], noPrevStat: Boolean = false, what: String = "statement", altEnd: Token = EOF): Boolean =
      def recur(sepSeen: Boolean, endSeen: Boolean): Boolean =
        if isStatSep then
          in.nextToken()
          recur(true, endSeen)
        else if in.token == END then
          if endSeen then syntaxError(em"duplicate end marker")
          checkEndMarker(stats)
          recur(sepSeen, endSeen = true)
        else if isStatSeqEnd || in.token == altEnd then
          false
        else if sepSeen || endSeen then
          true
        else
          val found = in.token
          val statFollows = mustStartStatTokens.contains(found)
          syntaxError(
            if noPrevStat then IllegalStartOfStatement(what, isModifier, statFollows)
            else em"end of $what expected but ${showToken(found)} found")
          if mustStartStatTokens.contains(found) then
            false // it's a statement that might be legal in an outer context
          else
            in.nextToken() // needed to ensure progress; otherwise we might cycle forever
            skip()
            true

      in.observeOutdented()
      recur(false, false)
    end statSepOrEnd

    def rewriteNotice(version: SourceVersion = `3.0-migration`, additionalOption: String = "") =
      Message.rewriteNotice("This construct", version, additionalOption)

    def syntaxVersionError(option: String, span: Span) =
      syntaxError(em"""This construct is not allowed under $option.${rewriteNotice(`3.0-migration`, option)}""", span)

    def rewriteToNewSyntax(span: Span = Span(in.offset)): Boolean = {
      if (in.newSyntax) {
        if (in.rewrite) return true
        syntaxVersionError("-new-syntax", span)
      }
      false
    }

    def rewriteToOldSyntax(span: Span = Span(in.offset)): Boolean = {
      if (in.oldSyntax) {
        if (in.rewrite) return true
        syntaxVersionError("-old-syntax", span)
      }
      false
    }

    def errorTermTree(start: Offset): Tree = atSpan(start, in.offset, in.offset) { unimplementedExpr }

    private var inFunReturnType = false
    private def fromWithinReturnType[T](body: => T): T = {
      val saved = inFunReturnType
      try {
        inFunReturnType = true
        body
      }
      finally inFunReturnType = saved
    }

    /** A flag indicating we are parsing in the annotations of a primary
     *  class constructor
     */
    private var inClassConstrAnnots = false
    private def fromWithinClassConstr[T](body: => T): T = {
      val saved = inClassConstrAnnots
      inClassConstrAnnots = true
      try body
      finally inClassConstrAnnots = saved
    }

    private var inEnum = false
    private def withinEnum[T](body: => T): T = {
      val saved = inEnum
      inEnum = true
      try body
      finally inEnum = saved
    }

    private var staged = StageKind.None
    def withinStaged[T](kind: StageKind)(op: => T): T = {
      val saved = staged
      staged |= kind
      try op
      finally staged = saved
    }

/* ---------- TREE CONSTRUCTION ------------------------------------------- */

    /** Convert tree to formal parameter list
    */
    def convertToParams(tree: Tree): List[ValDef] =
      val mods =
        if in.token == CTXARROW || isPureArrow(nme.PURECTXARROW)
        then Modifiers(Given)
        else EmptyModifiers
      tree match
        case Parens(t) =>
          convertToParam(t, mods) :: Nil
        case Tuple(ts) =>
          ts.map(convertToParam(_, mods))
        case t @ Typed(Ident(_), _) =>
          report.errorOrMigrationWarning(
            em"parentheses are required around the parameter of a lambda${rewriteNotice()}",
            in.sourcePos(), from = `3.0`)
          if sourceVersion.isMigrating then
            patch(source, t.span.startPos, "(")
            patch(source, t.span.endPos, ")")
          convertToParam(t, mods) :: Nil
        case t =>
          convertToParam(t, mods) :: Nil

    /** Convert tree to formal parameter
    */
    def convertToParam(tree: Tree, mods: Modifiers): ValDef =
      def fail() =
        syntaxError(em"not a legal formal parameter for a function literal", tree.span)
        makeParameter(nme.ERROR, tree, mods)
      tree match
        case param: ValDef =>
          param.withMods(param.mods | mods.flags)
        case id @ Ident(name) =>
          makeParameter(name.asTermName, TypeTree(), mods, isBackquoted = isBackquoted(id)).withSpan(tree.span)
        // the following three cases are needed only for 2.x parameters without enclosing parentheses
        case Typed(_, tpt: TypeBoundsTree) =>
          fail()
        case Typed(id @ Ident(name), tpt) =>
          makeParameter(name.asTermName, tpt, mods, isBackquoted = isBackquoted(id)).withSpan(tree.span)
        case _ =>
          fail()

    /** Checks that tuples don't contain a parameter. */
    def checkNonParamTuple(t: Tree) = t match
      case Tuple(ts) => ts.collectFirst {
        case param: ValDef =>
          syntaxError(em"invalid parameter definition syntax in tuple value", param.span)
      }
      case _ =>


    /** Convert (qual)ident to type identifier
     */
    def convertToTypeId(tree: Tree): Tree = tree match {
      case id @ Ident(name) =>
        cpy.Ident(id)(name.toTypeName)
      case id @ Select(qual, name) =>
        cpy.Select(id)(qual, name.toTypeName)
      case _ =>
        syntaxError(IdentifierExpected(tree.show), tree.span)
        tree
    }

/* --------------- PLACEHOLDERS ------------------------------------------- */

    /** The implicit parameters introduced by `_` in the current expression.
     *  Parameters appear in reverse order.
     */
    var placeholderParams: List[ValDef] = Nil

    def checkNoEscapingPlaceholders[T](op: => T): T =
      val savedPlaceholderParams = placeholderParams
      val savedLanguageImportContext = in.languageImportContext
      placeholderParams = Nil
      try op
      finally
        placeholderParams match
          case vd :: _ => syntaxError(UnboundPlaceholderParameter(), vd.span)
          case _ =>
        placeholderParams = savedPlaceholderParams
        in.languageImportContext = savedLanguageImportContext

    def isWildcard(t: Tree): Boolean = t match {
      case Ident(name1) => placeholderParams.nonEmpty && name1 == placeholderParams.head.name
      case Typed(t1, _) => isWildcard(t1)
      case Annotated(t1, _) => isWildcard(t1)
      case Parens(t1) => isWildcard(t1)
      case _ => false
    }

    def isWildcardType(t: Tree): Boolean = t match {
      case t: TypeBoundsTree => true
      case Parens(t1) => isWildcardType(t1)
      case _ => false
    }

    def rejectWildcardType(t: Tree, fallbackTree: Tree = scalaAny): Tree =
      if (isWildcardType(t)) {
        syntaxError(UnboundWildcardType(), t.span)
        fallbackTree
      }
      else t

/* -------------- XML ---------------------------------------------------- */

    /** The markup parser.
     *  The first time this lazy val is accessed, we assume we were trying to parse an XML literal.
     *  The current position is recorded for later error reporting if it turns out
     *  that we don't have scala-xml on the compilation classpath.
     */
    lazy val xmlp: xml.MarkupParsers.MarkupParser = {
      myFirstXmlPos = source.atSpan(Span(in.offset))
      new MarkupParser(this, true)
    }

    /** The position of the first XML literal encountered while parsing,
     *  NoSourcePosition if there were no XML literals.
     */
    def firstXmlPos: SourcePosition = myFirstXmlPos
    private var myFirstXmlPos: SourcePosition = NoSourcePosition

    object symbXMLBuilder extends xml.SymbolicXMLBuilder(this, true) // DEBUG choices

    def xmlLiteral() : Tree = xmlp.xLiteral
    def xmlLiteralPattern() : Tree = xmlp.xLiteralPattern

/* -------- COMBINATORS -------------------------------------------------------- */

    def enclosed[T](tok: Token, body: => T): T =
      accept(tok)
      try body finally accept(tok + 1)

    /** Same as enclosed, but if closing token is missing, add `,` to the expected tokens
     *  in the error message provided the next token could have followed a `,`.
     */
    def enclosedWithCommas[T](tok: Token, body: => T): T =
      accept(tok)
      val closing = tok + 1
      val isEmpty = in.token == closing
      val ts = body
      if in.token != closing then
        val followComma =
          if tok == LPAREN then canStartExprTokens3 else canStartTypeTokens
        val prefix = if !isEmpty && followComma.contains(in.token) then "',' or " else ""
        syntaxErrorOrIncomplete(ExpectedTokenButFound(closing, in.token, prefix))
      if in.token == closing then in.nextToken()
      ts

    def inParens[T](body: => T): T = enclosed(LPAREN, body)
    def inBraces[T](body: => T): T = enclosed(LBRACE, body)
    def inBrackets[T](body: => T): T = enclosed(LBRACKET, body)

    def inParensWithCommas[T](body: => T): T = enclosedWithCommas(LPAREN, body)
    def inBracketsWithCommas[T](body: => T): T = enclosedWithCommas(LBRACKET, body)

    def inBracesOrIndented[T](body: => T, rewriteWithColon: Boolean = false): T =
      if in.token == INDENT then
        val rewriteToBraces = in.rewriteNoIndent
          && !testChars(in.lastOffset - 3, " =>") // braces are always optional after `=>` so none should be inserted
        if rewriteToBraces then indentedToBraces(body)
        else enclosed(INDENT, body)
      else
        if in.rewriteToIndent then bracesToIndented(body, rewriteWithColon)
        else inBraces(body)

    def inDefScopeBraces[T](body: => T, rewriteWithColon: Boolean = false): T =
      inBracesOrIndented(body, rewriteWithColon)

    /** <part> {`,` <part>} */
    def commaSeparated[T](part: () => T): List[T] =
      in.currentRegion.withCommasExpected {
        commaSeparatedRest(part(), part)
      }

    /** {`,` <part>}
     *
     *  currentRegion.commasExpected has to be set separately.
     */
    def commaSeparatedRest[T](leading: T, part: () => T): List[T] =
      if in.token == COMMA then
        val ts = new ListBuffer[T] += leading
        while in.token == COMMA do
          in.nextToken()
          ts += part()
        ts.toList
      else leading :: Nil

    def inSepRegion[T](f: Region => Region)(op: => T): T =
      val cur = in.currentRegion
      in.currentRegion = f(cur)
      try op finally in.currentRegion = cur

    /** Parse `body` while checking (under -no-indent) that a `{` is not missing before it.
     *  This is done as follows:
     *    If the next token S is indented relative to the current region,
     *    and the end of `body` is followed by a new line and another statement,
     *    check that that other statement is indented less than S
     */
    def subPart[T](body: () => T): T = in.currentRegion match
      case r: InBraces if in.isAfterLineEnd =>
        val startIndentWidth = in.indentWidth(in.offset)
        if r.indentWidth < startIndentWidth then
          // Note: we can get here only if indentation is not significant
          // If indentation is significant, we would see an <indent> as current token
          // and the indent region would be Indented instead of InBraces.
          //
          // If indentation would be significant, an <indent> would be inserted here.
          val t = body()
          // Therefore, make sure there would be a matching <outdent>
          def nextIndentWidth = in.indentWidth(in.next.offset)
          if in.isNewLine && !(nextIndentWidth < startIndentWidth) then
            warning(
              if startIndentWidth <= nextIndentWidth then
                em"""Line is indented too far to the right, or a `{` is missing before:
                  |
                  |${t.tryToShow}"""
              else
                in.spaceTabMismatchMsg(startIndentWidth, nextIndentWidth),
              in.next.offset
            )
          t
        else body()
      case _ => body()

    /** Check that this is not the start of a statement that's indented relative to the current region.
     */
    def checkNextNotIndented(): Unit =
      if in.isNewLine then
        val nextIndentWidth = in.indentWidth(in.next.offset)
        if in.currentRegion.indentWidth < nextIndentWidth then
          warning(em"Line is indented too far to the right, or a `{` or `:` is missing", in.next.offset)

/* -------- REWRITES ----------------------------------------------------------- */

    /** The last offset where a colon at the end of line would be required if a subsequent { ... }
     *  block would be converted to an indentation region.
     */
    var possibleColonOffset: Int = -1

    def testChar(idx: Int, p: Char => Boolean): Boolean = {
      val txt = source.content
      idx < txt.length && p(txt(idx))
    }

    def testChar(idx: Int, c: Char): Boolean = {
      val txt = source.content
      idx < txt.length && txt(idx) == c
    }

    def testChars(from: Int, str: String): Boolean =
      str.isEmpty ||
      testChar(from, str.head) && testChars(from + 1, str.tail)

    def skipBlanks(idx: Int, step: Int = 1): Int =
      if (testChar(idx, c => c == ' ' || c == '\t' || c == Chars.CR)) skipBlanks(idx + step, step)
      else idx

    /** Parse indentation region `body` and rewrite it to be in braces instead */
    def indentedToBraces[T](body: => T): T =
      val enclRegion   = in.currentRegion.enclosing          // capture on entry
      def indentWidth  = enclRegion.indentWidth
      val followsColon = testChar(in.lastOffset - 1, ':')

      /** Is `expr` a tree that lacks a final `else`? Put such trees in `{...}` to make
       *  sure we don't accidentally merge them with a following `else`.
       */
      def isPartialIf(expr: Tree): Boolean = expr match {
        case If(_, _, EmptyTree) => true
        case If(_, _, e)         => isPartialIf(e)
        case _                   => false
      }

      /** Is `expr` a (possibly curried) function that has a multi-statement block
       *  as body? Put such trees in `{...}` since we don't enclose statements following
       *  a `=>` in braces.
       */
      def isBlockFunction[T](expr: T): Boolean = expr match {
        case Function(_, body)  => isBlockFunction(body)
        case Block(stats, expr) => stats.nonEmpty || isBlockFunction(expr)
        case _                  => false
      }

      /** Start of first line after in.lastOffset that does not have a comment
       *  at indent width greater than the indent width of the closing brace.
       */
      def closingOffset(lineStart: Offset): Offset =
        if in.lineOffset >= 0 && lineStart >= in.lineOffset then in.lineOffset
        else
          val commentStart = skipBlanks(lineStart)
          if testChar(commentStart, '/') && indentWidth < in.indentWidth(commentStart)
          then closingOffset(source.nextLine(lineStart))
          else lineStart

      def needsBraces(t: Any): Boolean = t match {
        case Match(EmptyTree, _) => true
        case Block(stats, expr)  => stats.nonEmpty || needsBraces(expr)
        case expr: Tree          => followsColon
                                 || isPartialIf(expr) && in.token == ELSE
                                 || isBlockFunction(expr)
        case _                   => true
      }
      // begin indentedToBraces
      val startOpening =
        if followsColon then
          if testChar(in.lastOffset - 2, ' ') then in.lastOffset - 2
          else in.lastOffset - 1
        else in.lastOffset
      val endOpening = in.lastOffset
      val t = enclosed(INDENT, body)
      if needsBraces(t) then
        patch(source, Span(startOpening, endOpening), " {")
        val next = in.next
        def closedByEndMarker =
          next.token == END && (next.offset - next.lineOffset) == indentWidth.toPrefix.size
        if closedByEndMarker then patch(source, Span(next.offset), "} // ")
        else patch(source, Span(closingOffset(source.nextLine(in.lastOffset))), indentWidth.toPrefix ++ "}\n")
      t
    end indentedToBraces

    /** The region to eliminate when replacing an opening `(` or `{` that ends a line.
     *  The `(` or `{` is at in.offset.
     */
    def startingElimRegion(colonRequired: Boolean): (Offset, Offset) = {
      val skipped = skipBlanks(in.offset + 1)
      if (in.isAfterLineEnd)
        if (testChar(skipped, Chars.LF) && !colonRequired)
          (in.lineOffset, skipped + 1) // skip the whole line
        else
          (in.offset, skipped)
      else if (testChar(in.offset - 1, ' ')) (in.offset - 1, in.offset + 1)
      else (in.offset, in.offset + 1)
    }

    /** The region to eliminate when replacing a closing `)` or `}` that starts a new line
     *  The `)` or `}` precedes in.lastOffset.
     */
    def closingElimRegion(): (Offset, Offset) = {
      val skipped = skipBlanks(in.lastOffset)
      if (testChar(skipped, Chars.LF))                    // if `)` or `}` is on a line by itself
        (source.startOfLine(in.lastOffset), skipped + 1)  //   skip the whole line
      else                                                // else
        (in.lastOffset - 1, skipped)                      //   move the following text up to where the `)` or `}` was
    }

    /** Parse brace-enclosed `body` and rewrite it to be an indentation region instead, if possible.
     *  If possible means:
     *   1. not inside (...), [...], case ... =>
     *   2. opening brace `{` is at end of line
     *   3. closing brace `}` is at start of line
     *   4. there is at least one token between the braces
     *   5. the closing brace is also at the end of the line, or it is followed by one of
     *      `then`, `else`, `do`, `catch`, `finally`, `yield`, or `match`.
     *   6. the opening brace does not follow a `=>`. The reason for this condition is that
     *      rewriting back to braces does not work after `=>` (since in most cases braces are omitted
     *      after a `=>` it would be annoying if braces were inserted).
     */
    def bracesToIndented[T](body: => T, rewriteWithColon: Boolean): T = {
      val underColonSyntax = possibleColonOffset == in.lastOffset
      val colonRequired = rewriteWithColon || underColonSyntax
      val (startOpening, endOpening) = startingElimRegion(colonRequired)
      val isOutermost = in.currentRegion.isOutermost
      def allBraces(r: Region): Boolean = r match {
        case r: Indented => r.isOutermost || allBraces(r.enclosing)
        case r: InBraces => allBraces(r.enclosing)
        case _ => false
      }
      var canRewrite = allBraces(in.currentRegion) && // test (1)
        !testChars(in.lastOffset - 3, " =>") // test(6)
      val t = enclosed(LBRACE, {
        canRewrite &= in.isAfterLineEnd // test (2)
        val curOffset = in.offset
        try body
        finally {
          canRewrite &= in.isAfterLineEnd && in.offset != curOffset // test (3)(4)
        }
      })
      canRewrite &= (in.isAfterLineEnd || statCtdTokens.contains(in.token)) // test (5)
      if canRewrite && (!underColonSyntax || Feature.fewerBracesEnabled) then
        val openingPatchStr =
          if !colonRequired then ""
          else if testChar(startOpening - 1, Chars.isOperatorPart(_)) then " :"
          else ":"
        val (startClosing, endClosing) = closingElimRegion()
        patch(source, Span(startOpening, endOpening), openingPatchStr)
        patch(source, Span(startClosing, endClosing), "")
      t
    }

    /** Drop (...) or { ... }, replacing the closing element with `endStr` */
    def dropParensOrBraces(start: Offset, endStr: String): Unit = {
      if (testChar(start + 1, Chars.isLineBreakChar))
        patch(source, Span(if (testChar(start - 1, ' ')) start - 1 else start, start + 1), "")
      else
        patch(source, Span(start, start + 1),
          if (testChar(start - 1, Chars.isIdentifierPart)) " " else "")
      val closingStartsLine = testChar(skipBlanks(in.lastOffset - 2, -1), Chars.LF)
      val preFill = if (closingStartsLine || endStr.isEmpty) "" else " "
      val postFill = if (in.lastOffset == in.offset) " " else ""
      val (startClosing, endClosing) =
        if (closingStartsLine && endStr.isEmpty) closingElimRegion()
        else (in.lastOffset - 1, in.lastOffset)
      patch(source, Span(startClosing, endClosing), s"$preFill$endStr$postFill")
    }

    /** If all other characters on the same line as `span` are blanks, widen to
     *  the whole line.
     */
    def widenIfWholeLine(span: Span): Span = {
      val start = skipBlanks(span.start - 1, -1)
      val end = skipBlanks(span.end, 1)
      if (testChar(start, Chars.LF) && testChar(end, Chars.LF)) Span(start, end)
      else span
    }

    /** Drop current token, if it is a `then` or `do`. */
    def dropTerminator(): Unit =
      if in.token == THEN || in.token == DO then
        var startOffset = in.offset
        var endOffset = in.lastCharOffset
        if (in.isAfterLineEnd) {
          if (testChar(endOffset, ' '))
            endOffset += 1
        }
        else
          if (testChar(startOffset - 1, ' ') &&
              !overlapsPatch(source, Span(startOffset - 1, endOffset)))
            startOffset -= 1
        patch(source, widenIfWholeLine(Span(startOffset, endOffset)), "")

    /** rewrite code with (...) around the source code of `t` */
    def revertToParens(t: Tree): Unit =
      if (t.span.exists) {
        patch(source, t.span.startPos, "(")
        patch(source, t.span.endPos, ")")
        dropTerminator()
      }

/* --------- LOOKAHEAD --------------------------------------- */

    /** In the tokens following the current one, does `query` precede any of the tokens that
     *   - must start a statement, or
     *   - separate two statements, or
     *   - continue a statement (e.g. `else`, catch`), or
     *   - terminate the current scope?
     */
    def followedByToken(query: Token): Boolean = {
      val lookahead = in.LookaheadScanner()
      var braces = 0
      while (true) {
        val token = lookahead.token
        if (query != LARROW && token == XMLSTART) return false
        if (braces == 0) {
          if (token == query) return true
          if (stopScanTokens.contains(token) || lookahead.isNestedEnd) return false
        }
        else if (token == EOF)
          return false
        else if (lookahead.isNestedEnd)
          braces -= 1
        if (lookahead.isNestedStart) braces += 1
        lookahead.nextToken()
      }
      false
    }

    /** Is the following sequence the generators of a for-expression enclosed in (...)? */
    def followingIsEnclosedGenerators(): Boolean = {
      val lookahead = in.LookaheadScanner()
      var parens = 1
      lookahead.nextToken()
      while (parens != 0 && lookahead.token != EOF) {
        val token = lookahead.token
        if (token == XMLSTART) return true
        if (token == LPAREN) parens += 1
        else if (token == RPAREN) parens -= 1
        lookahead.nextToken()
      }
      if (lookahead.token == LARROW)
        false // it's a pattern
      else if (lookahead.isIdent)
        true // it's not a pattern since token cannot be an infix operator
      else
        followedByToken(LARROW) // `<-` comes before possible statement starts
    }

    /** Are the next token the "GivenSig" part of a given definition,
     *  i.e. an identifier followed by type and value parameters, followed by `:`?
     *  @pre  The current token is an identifier
     */
    def followingIsGivenSig() =
      val lookahead = in.LookaheadScanner()
      if lookahead.isIdent then
        lookahead.nextToken()
      def skipParams(): Unit =
        if lookahead.token == LPAREN || lookahead.token == LBRACKET then
          lookahead.skipParens()
          skipParams()
        else if lookahead.isNewLine then
          lookahead.nextToken()
          skipParams()
      skipParams()
      lookahead.isColon

    def followingIsExtension() =
      val next = in.lookahead.token
      next == LBRACKET || next == LPAREN

    def followingIsSelfType() =
      val lookahead = in.LookaheadScanner(allowIndent = true)
      lookahead.nextToken()
      lookahead.token == COLONfollow
      && {
        lookahead.observeColonEOL(inTemplate = false)
        lookahead.nextToken()
        canStartInfixTypeTokens.contains(lookahead.token)
      }

    /** Is current ident a `*`, and is it followed by a `)`, `, )`, `,EOF`? The latter two are not
        syntactically valid, but we need to include them here for error recovery. */
    def followingIsVararg(): Boolean =
      in.isIdent(nme.raw.STAR) && {
        val lookahead = in.LookaheadScanner()
        lookahead.nextToken()
        lookahead.token == RPAREN
        || lookahead.token == COMMA
           && {
             lookahead.nextToken()
             lookahead.token == RPAREN || lookahead.token == EOF
           }
      }

    /** When encountering a `:`, is that in the binding of a lambda?
     *  @pre location of the enclosing expression is `InParens`, so there is an open `(`.
     */
    def followingIsLambdaParams() =
      val lookahead = in.LookaheadScanner()
      lookahead.nextToken()
      while lookahead.token != RPAREN && lookahead.token != EOF do
        if lookahead.token == LPAREN then lookahead.skipParens()
        else lookahead.nextToken()
      lookahead.token == RPAREN
      && {
        lookahead.nextToken()
        lookahead.isArrow
      }

    /** Is the token sequence following the current `:` token classified as a lambda?
     *  This is the case if the input starts with an identifier, a wildcard, or
     *  something enclosed in (...) or [...], and this is followed by a `=>` or `?=>`
     *  and an INDENT.
     */
    def followingIsLambdaAfterColon(): Boolean =
      val lookahead = in.LookaheadScanner(allowIndent = true)
      def isArrowIndent() =
        lookahead.isArrow
        && {
          lookahead.nextToken()
          lookahead.token == INDENT || lookahead.token == EOF
        }
      lookahead.nextToken()
      if lookahead.isIdent || lookahead.token == USCORE then
        lookahead.nextToken()
        isArrowIndent()
      else if lookahead.token == LPAREN || lookahead.token == LBRACKET then
        lookahead.skipParens()
        isArrowIndent()
      else false

    /** Can the next lookahead token start an operand as defined by
     *  leadingOperandTokens, or is postfix ops enabled?
     *  This is used to decide whether the current token can be an infix operator.
     */
    def nextCanFollowOperator(leadingOperandTokens: BitSet): Boolean =
      leadingOperandTokens.contains(in.lookahead.token)
      || in.postfixOpsEnabled
      || in.lookahead.token == COLONop
      || in.lookahead.token == EOF     // important for REPL completions
      || ctx.mode.is(Mode.Interactive) // in interactive mode the next tokens might be missing

  /* --------- OPERAND/OPERATOR STACK --------------------------------------- */

    var opStack: List[OpInfo] = Nil

    def checkAssoc(offset: Token, op1: Name, op2: Name, op2LeftAssoc: Boolean): Unit =
      if (op1.isRightAssocOperatorName == op2LeftAssoc)
        syntaxError(MixedLeftAndRightAssociativeOps(op1, op2, op2LeftAssoc), offset)

    def reduceStack(base: List[OpInfo], top: Tree, prec: Int, leftAssoc: Boolean, op2: Name, isType: Boolean): Tree = {
      if (opStack != base && precedence(opStack.head.operator.name) == prec)
        checkAssoc(opStack.head.offset, opStack.head.operator.name, op2, leftAssoc)
      def recur(top: Tree): Tree =
        if (opStack == base) top
        else {
          val opInfo = opStack.head
          val opPrec = precedence(opInfo.operator.name)
          if (prec < opPrec || leftAssoc && prec == opPrec) {
            opStack = opStack.tail
            recur {
              atSpan(opInfo.operator.span union opInfo.operand.span union top.span) {
                InfixOp(opInfo.operand, opInfo.operator, top)
              }
            }
          }
          else top
        }
      recur(top)
    }

    /** True if we are seeing a lambda argument after a colon of the form:
     *    : (params) =>
     *      body
     */
    def isColonLambda =
      Feature.fewerBracesEnabled && in.token == COLONfollow && followingIsLambdaAfterColon()

    /**   operand { infixop operand | MatchClause } [postfixop],
     *
     *  respecting rules of associativity and precedence.
     *  @param isOperator    the current token counts as an operator.
     *  @param maybePostfix  postfix operators are allowed.
     */
    def infixOps(
        first: Tree, canStartOperand: Token => Boolean, operand: Location => Tree,
        location: Location,
        kind: ParseKind,
        isOperator: => Boolean): Tree =
      val base = opStack

      def recur(top: Tree): Tree =
        def isType = kind == ParseKind.Type
        def maybePostfix = kind == ParseKind.Expr && in.postfixOpsEnabled
        if isIdent && isOperator then
          val op = if isType then typeIdent() else termIdent()
          val top1 = reduceStack(base, top, precedence(op.name), !op.name.isRightAssocOperatorName, op.name, isType)
          opStack = OpInfo(top1, op, in.offset) :: opStack
          colonAtEOLOpt()
          newLineOptWhenFollowing(canStartOperand)
          if isColonLambda then
            in.nextToken()
            recur(expr(Location.InColonArg))
          else if maybePostfix && !canStartOperand(in.token) then
            val topInfo = opStack.head
            opStack = opStack.tail
            val od = reduceStack(base, topInfo.operand, 0, true, in.name, isType)
            atSpan(startOffset(od), topInfo.offset) {
              PostfixOp(od, topInfo.operator)
            }
          else recur(operand(location))
        else
          val t = reduceStack(base, top, minPrec, leftAssoc = true, in.name, isType)
          if !isType && in.token == MATCH then recurAtMinPrec(matchClause(t))
          else t

      def recurAtMinPrec(top: Tree): Tree =
        if isIdent && isOperator && precedence(in.name) == minInfixPrec
           || in.token == MATCH
        then recur(top)
        else top

      val res = recur(first)
      if isIdent(nme.raw.STAR) && !followingIsVararg() then
        syntaxError(em"spread operator `*` not allowed here; must come last in a parameter list")
        in.nextToken()
      res
    end infixOps

/* -------- IDENTIFIERS AND LITERALS ------------------------------------------- */

    /** Accept identifier and return its name as a term name. */
    def ident(): TermName =
      if (isIdent) {
        val name = in.name
        if name == nme.CONSTRUCTOR || name == nme.STATIC_CONSTRUCTOR then
          report.error(
            em"""Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden""",
            in.sourcePos())
        in.nextToken()
        name
      }
      else {
        syntaxErrorOrIncomplete(ExpectedTokenButFound(IDENTIFIER, in.token))
        nme.ERROR
      }

    /** Accept identifier and return Ident with its name as a term name. */
    def termIdent(): Ident =
      makeIdent(in.token, in.offset, ident())

    /** Accept identifier and return Ident with its name as a type name. */
    def typeIdent(): Ident =
      makeIdent(in.token, in.offset, ident().toTypeName)

    private def makeIdent(tok: Token, offset: Offset, name: Name) = {
      val tree = Ident(name)
      if (tok == BACKQUOTED_IDENT) tree.pushAttachment(Backquoted, ())

      // Make sure that even trees with parsing errors have a offset that is within the offset
      val errorOffset = offset min (in.lastOffset - 1)
      if (tree.name == nme.ERROR && tree.span == NoSpan) tree.withSpan(Span(errorOffset, errorOffset))
      else atSpan(offset)(tree)
    }

    def wildcardIdent(): Ident =
      atSpan(accept(USCORE)) { Ident(nme.WILDCARD) }

    /** Accept identifier or match clause acting as a selector on given tree `t` */
    def selectorOrMatch(t: Tree): Tree =
      atSpan(startOffset(t), in.offset) {
        if in.token == MATCH then matchClause(t) else Select(t, ident())
      }

    def selector(t: Tree): Tree =
      atSpan(startOffset(t), in.offset) { Select(t, ident()) }

    /** DotSelectors ::= { `.' id } */
    def dotSelectors(t: Tree): Tree =
      if (in.token == DOT) { in.nextToken(); dotSelectors(selector(t)) }
      else t

    private val id: Tree => Tree = x => x

    /** SimpleRef  ::= id
     *              |  [id ‘.’] ‘this’
     *              |  [id ‘.’] ‘super’ [ClassQualifier] ‘.’ id
     */
    def simpleRef(): Tree =
      val start = in.offset

      def handleThis(qual: Ident) =
        in.nextToken()
        atSpan(start) { This(qual) }

      def handleSuper(qual: Ident) =
        in.nextToken()
        val mix = mixinQualifierOpt()
        val t = atSpan(start) { Super(This(qual), mix) }
        accept(DOT)
        selector(t)

      if in.token == THIS then handleThis(EmptyTypeIdent)
      else if in.token == SUPER then handleSuper(EmptyTypeIdent)
      else
        val t = termIdent()
        if in.token == DOT then
          def qual = cpy.Ident(t)(t.name.toTypeName)
          in.lookahead.token match
            case THIS =>
              in.nextToken()
              handleThis(qual)
            case SUPER =>
              in.nextToken()
              handleSuper(qual)
            case _ => t
        else t
    end simpleRef

    /** MixinQualifier ::= `[' id `]'
    */
    def mixinQualifierOpt(): Ident =
      if (in.token == LBRACKET) inBrackets(atSpan(in.offset) { typeIdent() })
      else EmptyTypeIdent

    /** QualId ::= id {`.' id}
    */
    def qualId(): Tree = dotSelectors(termIdent())

    /** Singleton    ::=  SimpleRef
     *                 |  SimpleLiteral
     *                 |  Singleton ‘.’ id
     * -- not yet      |  Singleton ‘(’ Singletons ‘)’
     * -- not yet      |  Singleton ‘[’ Types ‘]’
     */
    def singleton(): Tree =
      if isSimpleLiteral then simpleLiteral()
      else dotSelectors(simpleRef())

    /** SimpleLiteral     ::=  [‘-’] integerLiteral
     *                      |  [‘-’] floatingPointLiteral
     *                      |  booleanLiteral
     *                      |  characterLiteral
     *                      |  stringLiteral
     */
    def simpleLiteral(): Tree =
      if isIdent(nme.raw.MINUS) then
        val start = in.offset
        in.nextToken()
        literal(negOffset = start, inTypeOrSingleton = true)
      else
        literal(inTypeOrSingleton = true)

    /** Literal           ::=  SimpleLiteral
     *                      |  processedStringLiteral
     *                      |  symbolLiteral
     *                      |  ‘null’
     *
     *  @param negOffset   The offset of a preceding `-' sign, if any.
     *                     If the literal is not negated, negOffset == in.offset.
     */
    def literal(negOffset: Int = in.offset, inPattern: Boolean = false, inTypeOrSingleton: Boolean = false, inStringInterpolation: Boolean = false): Tree = {
      def literalOf(token: Token): Tree = {
        val isNegated = negOffset < in.offset
        def digits0 = in.removeNumberSeparators(in.strVal)
        def digits = if (isNegated) "-" + digits0 else digits0
        if !inTypeOrSingleton then
          token match {
            case INTLIT  => return Number(digits, NumberKind.Whole(in.base))
            case DECILIT => return Number(digits, NumberKind.Decimal)
            case EXPOLIT => return Number(digits, NumberKind.Floating)
            case _ =>
          }
        import scala.util.FromDigits.*
        val value =
          try token match {
            case INTLIT                        => intFromDigits(digits, in.base)
            case LONGLIT                       => longFromDigits(digits, in.base)
            case FLOATLIT                      => floatFromDigits(digits)
            case DOUBLELIT | DECILIT | EXPOLIT => doubleFromDigits(digits)
            case CHARLIT                       => in.strVal.head
            case STRINGLIT | STRINGPART        => in.strVal
            case TRUE                          => true
            case FALSE                         => false
            case NULL                          => null
            case _                             =>
              syntaxErrorOrIncomplete(IllegalLiteral())
              null
          }
          catch {
            case ex: FromDigitsException => syntaxErrorOrIncomplete(ex.getMessage.toMessage)
          }
        Literal(Constant(value))
      }

      if (inStringInterpolation) {
        val t = in.token match {
          case STRINGLIT | STRINGPART =>
            val value = in.strVal
            atSpan(negOffset, negOffset, negOffset + value.length) { Literal(Constant(value)) }
          case _ =>
            syntaxErrorOrIncomplete(IllegalLiteral())
            atSpan(negOffset) { Literal(Constant(null)) }
        }
        in.nextToken()
        t
      }
      else atSpan(negOffset) {
        if (in.token == QUOTEID)
          if ((staged & StageKind.Spliced) != 0 && Chars.isIdentifierStart(in.name(0))) {
            val t = atSpan(in.offset + 1) {
              val tok = in.toToken(in.name)
              tok match {
                case TRUE | FALSE | NULL => literalOf(tok)
                case THIS => This(EmptyTypeIdent)
                case _ => Ident(in.name)
              }
            }
            in.nextToken()
            Quote(t, Nil)
          }
          else
            if !in.featureEnabled(Feature.symbolLiterals) then
              val name = in.name // capture name (not `in`) in the warning message closure
              report.errorOrMigrationWarning(
                em"""symbol literal '$name is no longer supported,
                    |use a string literal "$name" or an application Symbol("$name") instead,
                    |or enclose in braces '{$name} if you want a quoted expression.
                    |For now, you can also `import language.deprecated.symbolLiterals` to accept
                    |the idiom, but this possibility might no longer be available in the future.""",
                in.sourcePos(), from = `3.0`)
              if sourceVersion.isMigrating then
                patch(source, Span(in.offset, in.offset + 1), "Symbol(\"")
                patch(source, Span(in.charOffset - 1), "\")")
            atSpan(in.skipToken()) { SymbolLit(in.strVal) }
        else if (in.token == INTERPOLATIONID) interpolatedString(inPattern)
        else {
          val t = literalOf(in.token)
          in.nextToken()
          t
        }
      }
    }

    private def interpolatedString(inPattern: Boolean = false): Tree = atSpan(in.offset) {
      val segmentBuf = new ListBuffer[Tree]
      val interpolator = in.name
      val isTripleQuoted =
        in.charOffset + 1 < in.buf.length &&
        in.buf(in.charOffset) == '"' &&
        in.buf(in.charOffset + 1) == '"'
      in.nextToken()
      def nextSegment(literalOffset: Offset) =
        segmentBuf += Thicket(
            literal(literalOffset, inPattern = inPattern, inStringInterpolation = true),
            atSpan(in.offset) {
              if (in.token == IDENTIFIER)
                termIdent()
              else if (in.token == USCORE && inPattern) {
                in.nextToken()
                Ident(nme.WILDCARD)
              }
              else if (in.token == THIS) {
                in.nextToken()
                This(EmptyTypeIdent)
              }
              else if (in.token == LBRACE)
                if (inPattern) Block(Nil, inBraces(pattern()))
                else expr()
              else {
                report.error(InterpolatedStringError(), source.atSpan(Span(in.offset)))
                EmptyTree
              }
            })

      var offsetCorrection = if isTripleQuoted then 3 else 1
      while (in.token == STRINGPART)
        nextSegment(in.offset + offsetCorrection)
        offsetCorrection = 0
      if (in.token == STRINGLIT)
        segmentBuf += literal(inPattern = inPattern, negOffset = in.offset + offsetCorrection, inStringInterpolation = true)

      InterpolatedString(interpolator, segmentBuf.toList)
    }

/* ------------- NEW LINES ------------------------------------------------- */

    def newLineOpt(): Unit =
      if (in.token == NEWLINE) in.nextToken()

    def newLinesOpt(): Unit =
      if in.isNewLine then in.nextToken()

    def newLineOptWhenFollowedBy(token: Int): Unit =
      // note: next is defined here because current == NEWLINE
      if (in.token == NEWLINE && in.next.token == token) in.nextToken()

    def newLinesOptWhenFollowedBy(token: Int): Unit =
      if in.isNewLine && in.next.token == token then in.nextToken()

    def newLinesOptWhenFollowedBy(name: Name): Unit =
      if in.isNewLine && in.next.token == IDENTIFIER && in.next.name == name then
        in.nextToken()

    def newLineOptWhenFollowing(p: Int => Boolean): Unit =
      // note: next is defined here because current == NEWLINE
      if (in.token == NEWLINE && p(in.next.token)) newLineOpt()

    def acceptIndent() =
      if in.token != INDENT then
        syntaxErrorOrIncomplete(em"indented definitions expected, ${in} found")

    def colonAtEOLOpt(): Unit =
      possibleColonOffset = in.lastOffset
      in.observeColonEOL(inTemplate = false)
      if in.token == COLONeol then
        in.nextToken()
        acceptIndent()

    def argumentStart(): Unit =
      colonAtEOLOpt()
      if migrateTo3 && in.token == NEWLINE && in.next.token == LBRACE then
        in.nextToken()
        if in.indentWidth(in.offset) == in.currentRegion.indentWidth then
          report.errorOrMigrationWarning(
            em"""This opening brace will start a new statement in Scala 3.
                |It needs to be indented to the right to keep being treated as
                |an argument to the previous expression.${rewriteNotice()}""",
            in.sourcePos(), from = `3.0`)
          if sourceVersion.isMigrating then
            patch(source, Span(in.offset), "  ")

    def possibleTemplateStart(isNew: Boolean = false): Unit =
      in.observeColonEOL(inTemplate = true)
      if in.token == COLONeol then
        if in.lookahead.token == END then in.token = NEWLINE
        else
          in.nextToken()
          if in.token != LBRACE then acceptIndent()
      else
        newLineOptWhenFollowedBy(LBRACE)

    def checkEndMarker[T <: Tree](stats: ListBuffer[T]): Unit =

      def updateSpanOfLast(last: T): Unit =
        last match
          case last: WithEndMarker[t] => last.withEndMarker()
          case _ =>
        last.span = last.span.withEnd(in.lastCharOffset)

      def matches(stat: T): Boolean = stat match
        case stat: MemberDef if !stat.name.isEmpty =>
          if stat.name == nme.CONSTRUCTOR then in.token == THIS
          else in.isIdent && in.name == stat.name.toTermName
        case ExtMethods(_, _) =>
          in.token == IDENTIFIER && in.name == nme.extension
        case PackageDef(pid: RefTree, _) =>
          in.isIdent && in.name == pid.name
        case stat: MemberDef if stat.mods.is(Given) => in.token == GIVEN
        case _: PatDef => in.token == VAL
        case _: If => in.token == IF
        case _: WhileDo => in.token == WHILE
        case _: ParsedTry => in.token == TRY
        case _: Match => in.token == MATCH
        case _: New => in.token == NEW
        case _: (ForYield | ForDo) => in.token == FOR
        case _ => false

      def endName = if in.token == IDENTIFIER then in.name.toString else tokenString(in.token)

      def matchesAndSetEnd(last: T): Boolean =
        val didMatch = matches(last)
        if didMatch then
          updateSpanOfLast(last)
        didMatch

      if in.token == END then
        val start = in.skipToken()
        if stats.isEmpty || !matchesAndSetEnd(stats.last) then
          syntaxError(em"misaligned end marker", Span(start, in.lastCharOffset))
        in.token = IDENTIFIER // Leaving it as the original token can confuse newline insertion
        in.nextToken()
    end checkEndMarker

/* ------------- TYPES ------------------------------------------------------ */

    /** Same as [[typ]], but if this results in a wildcard it emits a syntax error and
     *  returns a tree for type `Any` instead.
     */
    def toplevelTyp(): Tree = rejectWildcardType(typ())

    private def getFunction(tree: Tree): Option[Function] = tree match {
      case Parens(tree1) => getFunction(tree1)
      case Block(Nil, tree1) => getFunction(tree1)
      case t: Function => Some(t)
      case _ => None
    }

    private def checkFunctionNotErased(f: Function, context: String) =
      def fail(span: Span) =
        syntaxError(em"Implementation restriction: erased parameters are not supported in $context", span)
      // erased parameter in type
      val hasErasedParam = f match
        case f: FunctionWithMods => f.hasErasedParams
        case _ => false
      if hasErasedParam then
        fail(f.span)
      // erased parameter in term
      val hasErasedMods = f.args.collectFirst {
        case v: ValDef if v.mods.is(Flags.Erased) => v
      }
      hasErasedMods match
        case Some(param) => fail(param.span)
        case _ =>

    /** CaptureRef  ::=  ident | `this`
     */
    def captureRef(): Tree =
      if in.token == THIS then simpleRef()
      else termIdent() match
        case Ident(nme.CAPTURE_ROOT) => captureRoot
        case id => id

    /**  CaptureSet ::=  `{` CaptureRef {`,` CaptureRef} `}`    -- under captureChecking
     */
    def captureSet(): List[Tree] = inBraces {
      if in.token == RBRACE then Nil else commaSeparated(captureRef)
    }

    def capturesAndResult(core: () => Tree): Tree =
      if Feature.ccEnabled && in.token == LBRACE && in.offset == in.lastOffset
      then CapturesAndResult(captureSet(), core())
      else core()

    /** Type           ::=  FunType
     *                   |  HkTypeParamClause ‘=>>’ Type
     *                   |  FunParamClause ‘=>>’ Type
     *                   |  MatchType
     *                   |  InfixType
     *  FunType        ::=  (MonoFunType | PolyFunType)
     *  MonoFunType    ::=  FunTypeArgs (‘=>’ | ‘?=>’) Type
     *                   |  (‘->’ | ‘?->’ ) [CaptureSet] Type          -- under pureFunctions
     *  PolyFunType    ::=  HKTypeParamClause '=>' Type
     *                   |  HKTypeParamClause ‘->’ [CaptureSet] Type   -- under pureFunctions
     *  FunTypeArgs    ::=  InfixType
     *                   |  `(' [ FunArgType {`,' FunArgType } ] `)'
     *                   |  '(' [ TypedFunParam {',' TypedFunParam } ')'
     *  MatchType      ::=  InfixType `match` <<< TypeCaseClauses >>>
     */
    def typ(): Tree =
      val start = in.offset
      var imods = Modifiers()
      val erasedArgs: ListBuffer[Boolean] = ListBuffer()

      def functionRest(params: List[Tree]): Tree =
        val paramSpan = Span(start, in.lastOffset)
        atSpan(start, in.offset) {
          var token = in.token
          var isPure = false
          if isPureArrow(nme.PUREARROW) then
            isPure = true
            token = ARROW
          else if isPureArrow(nme.PURECTXARROW) then
            isPure = true
            token = CTXARROW
          else if token == TLARROW then
            if !imods.flags.isEmpty || params.isEmpty then
              syntaxError(em"illegal parameter list for type lambda", start)
              token = ARROW
          else if Feature.pureFunsEnabled then
            // `=>` means impure function under pureFunctions or captureChecking
            // language imports, whereas `->` is then a regular function.
            imods |= Impure

          if token == CTXARROW then
            in.nextToken()
            imods |= Given
          else if token == ARROW || token == TLARROW then
            in.nextToken()
          else
            accept(ARROW)

          val resultType =
            if isPure then capturesAndResult(typ) else typ()
          if token == TLARROW then
            for case ValDef(_, tpt, _) <- params do
              if isByNameType(tpt) then
                syntaxError(em"parameter of type lambda may not be call-by-name", tpt.span)
            TermLambdaTypeTree(params.asInstanceOf[List[ValDef]], resultType)
          else if imods.isOneOf(Given | Impure) || erasedArgs.contains(true) then
            if imods.is(Given) && params.isEmpty then
              syntaxError(em"context function types require at least one parameter", paramSpan)
            FunctionWithMods(params, resultType, imods, erasedArgs.toList)
          else if !ctx.settings.YkindProjector.isDefault then
            val (newParams :+ newResultType, tparams) = replaceKindProjectorPlaceholders(params :+ resultType): @unchecked
            lambdaAbstract(tparams, Function(newParams, newResultType))
          else
            Function(params, resultType)
        }

      def typeRest(t: Tree) = in.token match
        case ARROW | CTXARROW =>
          erasedArgs.addOne(false)
          functionRest(t :: Nil)
        case MATCH =>
          matchType(t)
        case FORSOME =>
          syntaxError(ExistentialTypesNoLongerSupported())
          t
        case _ if isPureArrow =>
          erasedArgs.addOne(false)
          functionRest(t :: Nil)
        case _ =>
          if erasedArgs.contains(true) && !t.isInstanceOf[FunctionWithMods] then
            syntaxError(ErasedTypesCanOnlyBeFunctionTypes(), implicitKwPos(start))
          t

      var isValParamList = false
      if in.token == LPAREN then
        in.nextToken()
        if in.token == RPAREN then
          in.nextToken()
          functionRest(Nil)
        else
          val paramStart = in.offset
          def addErased() =
            erasedArgs.addOne(isErasedKw)
            if isErasedKw then in.skipToken()
          addErased()
          val args =
            in.currentRegion.withCommasExpected:
              funArgType() match
                case Ident(name) if name != tpnme.WILDCARD && in.isColon =>
                  isValParamList = true
                  def funParam(start: Offset, mods: Modifiers) =
                    atSpan(start):
                      addErased()
                      typedFunParam(in.offset, ident(), imods)
                  commaSeparatedRest(
                    typedFunParam(paramStart, name.toTermName, imods),
                    () => funParam(in.offset, imods))
                case t =>
                  def funArg() =
                    erasedArgs.addOne(false)
                    funArgType()
                  commaSeparatedRest(t, funArg)
          accept(RPAREN)
          if isValParamList || in.isArrow || isPureArrow then
            functionRest(args)
          else
            val args1 = args.mapConserve: t =>
              if isByNameType(t) then
                syntaxError(ByNameParameterNotSupported(t), t.span)
                stripByNameType(t)
              else
                t
            val tuple = atSpan(start):
              makeTupleOrParens(args1)
            typeRest:
              infixTypeRest:
                refinedTypeRest:
                  withTypeRest:
                    annotTypeRest:
                      simpleTypeRest(tuple)
      else if in.token == LBRACKET then
        val start = in.offset
        val tparams = typeParamClause(ParamOwner.TypeParam)
        if in.token == TLARROW then
          atSpan(start, in.skipToken()):
            LambdaTypeTree(tparams, toplevelTyp())
        else if in.token == ARROW || isPureArrow(nme.PUREARROW) then
          val arrowOffset = in.skipToken()
          val body = toplevelTyp()
          atSpan(start, arrowOffset):
            getFunction(body) match
              case Some(f) =>
                checkFunctionNotErased(f, "poly function")
                PolyFunction(tparams, body)
              case None =>
                syntaxError(em"Implementation restriction: polymorphic function types must have a value parameter", arrowOffset)
                Ident(nme.ERROR.toTypeName)
        else
          accept(TLARROW)
          typ()
      else if in.token == INDENT then
        enclosed(INDENT, typ())
      else
        typeRest(infixType())
    end typ

    private def makeKindProjectorTypeDef(name: TypeName): TypeDef = {
      val isVarianceAnnotated = name.startsWith("+") || name.startsWith("-")
      // We remove the variance marker from the name without passing along the specified variance at all
      // The real variance will be inferred at a later stage but may contradict the variance specified,
      // This is ok, because `-Ykind-projector` is for cross-compiling existing Scala 2 code, not for writing new code,
      // we may assume that variance annotations have already been checked by the Scala 2 compiler.
      val unannotatedName = if (isVarianceAnnotated) name.mapLast(_.drop(1)) else name
      TypeDef(unannotatedName, WildcardTypeBoundsTree()).withFlags(Param)
    }

    /** Replaces kind-projector's `*` in a list of types arguments with synthetic names,
     *  returning the new argument list and the synthetic type definitions.
     */
    private def replaceKindProjectorPlaceholders(params: List[Tree]): (List[Tree], List[TypeDef]) = {
      val tparams = new ListBuffer[TypeDef]
      def addParam() = {
        val name = WildcardParamName.fresh().toTypeName
        tparams += makeKindProjectorTypeDef(name)
        Ident(name)
      }

      val uscores = ctx.settings.YkindProjector.value == "underscores"
      val newParams = params.mapConserve {
        case param @ Ident(tpnme.raw.STAR | tpnme.raw.MINUS_STAR | tpnme.raw.PLUS_STAR) => addParam()
        case param @ Ident(tpnme.USCOREkw | tpnme.raw.MINUS_USCORE | tpnme.raw.PLUS_USCORE) if uscores => addParam()
        case other => other
      }

      (newParams, tparams.toList)
    }

    private def implicitKwPos(start: Int): Span =
      Span(start, start + nme.IMPLICITkw.asSimpleName.length)

    /** TypedFunParam   ::= [`erased`] id ':' Type */
    def typedFunParam(start: Offset, name: TermName, mods: Modifiers = EmptyModifiers): ValDef =
      atSpan(start) {
        acceptColon()
        makeParameter(name, typ(), mods)
      }

    /**  FunParamClause ::=  ‘(’ TypedFunParam {‘,’ TypedFunParam } ‘)’
     */
    def funParamClause(): List[ValDef] =
      inParensWithCommas(commaSeparated(() => typedFunParam(in.offset, ident())))

    def funParamClauses(): List[List[ValDef]] =
      if in.token == LPAREN then funParamClause() :: funParamClauses() else Nil

    /** InfixType ::= RefinedType {id [nl] RefinedType}
     *             |  RefinedType `^`
     */
    def infixType(): Tree = infixTypeRest(refinedType())

    def infixTypeRest(t: Tree): Tree =
      infixOps(t, canStartInfixTypeTokens, refinedTypeFn, Location.ElseWhere, ParseKind.Type,
        isOperator = !followingIsVararg() && !isPureArrow
                     && nextCanFollowOperator(canStartInfixTypeTokens))

    /** RefinedType   ::=  WithType {[nl] Refinement} [`^` CaptureSet]
     */
    val refinedTypeFn: Location => Tree = _ => refinedType()

    def refinedType() = refinedTypeRest(withType())

    /** Disambiguation: a `^` is treated as a postfix operator meaning `^{cap}`
     *  if followed by `{`, `->`, or `?->`,
     *  or followed by a new line (significant or not),
     *  or followed by a token that cannot start an infix type.
     *  Otherwise it is treated as an infix operator.
     */
    private def isCaptureUpArrow =
      val ahead = in.lookahead
      ahead.token == LBRACE
      || ahead.isIdent(nme.PUREARROW)
      || ahead.isIdent(nme.PURECTXARROW)
      || !canStartInfixTypeTokens.contains(ahead.token)
      || ahead.lineOffset > 0

    def refinedTypeRest(t: Tree): Tree = {
      argumentStart()
      if in.isNestedStart then
        refinedTypeRest(atSpan(startOffset(t)) {
          RefinedTypeTree(rejectWildcardType(t), refinement(indentOK = true))
        })
      else if Feature.ccEnabled && in.isIdent(nme.UPARROW) && isCaptureUpArrow then
        val upArrowStart = in.offset
        in.nextToken()
        def cs =
          if in.token == LBRACE then captureSet()
          else atSpan(upArrowStart)(captureRoot) :: Nil
        makeRetaining(t, cs, tpnme.retains)
      else
        t
    }

    /** WithType ::= AnnotType {`with' AnnotType}    (deprecated)
     */
    def withType(): Tree = withTypeRest(annotType())

    def withTypeRest(t: Tree): Tree =
      if in.token == WITH then
        val withOffset = in.offset
        in.nextToken()
        if in.token == LBRACE || in.token == INDENT then
          t
        else
          if sourceVersion.isAtLeast(future) then
            deprecationWarning(DeprecatedWithOperator(), withOffset)
          atSpan(startOffset(t)) { makeAndType(t, withType()) }
      else t

    /** AnnotType ::= SimpleType {Annotation}
     */
    def annotType(): Tree = annotTypeRest(simpleType())

    def annotTypeRest(t: Tree): Tree =
      if (in.token == AT)
        annotTypeRest(atSpan(startOffset(t)) {
          Annotated(rejectWildcardType(t), annot())
        })
      else t

    /** The block in a quote or splice */
    def stagedBlock() = inBraces(block(simplify = true))

    /** ExprSplice  ::=  ‘$’ spliceId          --     if inside quoted block
     *                |  ‘$’ ‘{’ Block ‘}’     -- unless inside quoted pattern
     *                |  ‘$’ ‘{’ Pattern ‘}’   --   when inside quoted pattern
     *  TypeSplice  ::=  ‘$’ spliceId          --    if inside quoted type
     *                |  ‘$’ ‘{’ Block ‘}’     -- unless inside quoted type pattern
     *                |  ‘$’ ‘{’ Pattern ‘}’   --   when inside quoted type pattern
     */
    def splice(isType: Boolean): Tree =
      val start = in.offset
      atSpan(in.offset) {
        val inPattern = (staged & StageKind.QuotedPattern) != 0
        val expr =
          if (in.name.length == 1) {
            in.nextToken()
            withinStaged(StageKind.Spliced)(if (inPattern) inBraces(pattern()) else stagedBlock())
          }
          else atSpan(in.offset + 1) {
            val id = Ident(in.name.drop(1))
            in.nextToken()
            id
          }
        if isType then
          val msg = "Type splicing with `$` in quotes not supported anymore"
          val inPattern = (staged & StageKind.QuotedPattern) != 0
          val hint =
            if inPattern then "Use lower cased variable name without the `$` instead"
            else "To use a given Type[T] in a quote just write T directly"
          syntaxError(em"$msg\n\nHint: $hint", Span(start, in.lastOffset))
          Ident(nme.ERROR.toTypeName)
        else if inPattern then
          SplicePattern(expr, Nil)
        else
          Splice(expr)
      }

    /**  SimpleType      ::=  SimpleLiteral
     *                     |  ‘?’ SubtypeBounds
     *                     |  SimpleType1
     *                     |  SimpleType ‘(’ Singletons ‘)’  -- under language.experimental.dependent, checked in Typer
     *   Singletons      ::=  Singleton {‘,’ Singleton}
     */
    def simpleType(): Tree =
      if isSimpleLiteral then
        SingletonTypeTree(simpleLiteral())
      else if in.token == USCORE then
        if ctx.settings.YkindProjector.value == "underscores" then
          val start = in.skipToken()
          Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
        else
          if sourceVersion.isAtLeast(future) then
            deprecationWarning(em"`_` is deprecated for wildcard arguments of types: use `?` instead")
            patch(source, Span(in.offset, in.offset + 1), "?")
          val start = in.skipToken()
          typeBounds().withSpan(Span(start, in.lastOffset, start))
      // Allow symbols -_ and +_ through for compatibility with code written using kind-projector in Scala 3 underscore mode.
      // While these signify variant type parameters in Scala 2 + kind-projector, we ignore their variance markers since variance is inferred.
      else if (isIdent(nme.MINUS) || isIdent(nme.PLUS)) && in.lookahead.token == USCORE && ctx.settings.YkindProjector.value == "underscores" then
        val identName = in.name.toTypeName ++ nme.USCOREkw
        val start = in.skipToken()
        in.nextToken()
        Ident(identName).withSpan(Span(start, in.lastOffset, start))
      else if isIdent(nme.?) then
        val start = in.skipToken()
        typeBounds().withSpan(Span(start, in.lastOffset, start))
      else
        def singletonArgs(t: Tree): Tree =
          if in.token == LPAREN && in.featureEnabled(Feature.dependent)
          then singletonArgs(AppliedTypeTree(t, inParensWithCommas(commaSeparated(singleton))))
          else t
        singletonArgs(simpleType1())

    /** SimpleType1      ::=  id
     *                     |  Singleton `.' id
     *                     |  Singleton `.' type
     *                     |  ‘(’ ArgTypes ‘)’
     *                     |  Refinement
     *                     |  TypeSplice                -- deprecated syntax (since 3.0.0)
     *                     |  SimpleType1 TypeArgs
     *                     |  SimpleType1 `#' id
     */
    def simpleType1() = simpleTypeRest {
      if in.token == LPAREN then
        atSpan(in.offset) {
          makeTupleOrParens(inParensWithCommas(argTypes(namedOK = false, wildOK = true)))
        }
      else if in.token == LBRACE then
        atSpan(in.offset) { RefinedTypeTree(EmptyTree, refinement(indentOK = false)) }
      else if (isSplice)
        splice(isType = true)
      else
        def singletonCompletion(t: Tree): Tree =
          if in.token == DOT then
            in.nextToken()
            if in.token == TYPE then
              in.nextToken()
              atSpan(startOffset(t)) { SingletonTypeTree(t) }
            else
              singletonCompletion(selector(t))
          else convertToTypeId(t)
        singletonCompletion(simpleRef())
    }

    private def simpleTypeRest(t: Tree): Tree = in.token match {
      case HASH => simpleTypeRest(typeProjection(t))
      case LBRACKET => simpleTypeRest(atSpan(startOffset(t)) {
        val applied = rejectWildcardType(t)
        val args = typeArgs(namedOK = false, wildOK = true)

        if (!ctx.settings.YkindProjector.isDefault) {
          def fail(): Tree = {
            syntaxError(
              em"λ requires a single argument of the form X => ... or (X, Y) => ...",
              Span(startOffset(t), in.lastOffset)
            )
            AppliedTypeTree(applied, args)
          }

          applied match {
            case Ident(tpnme.raw.LAMBDA) =>
              args match {
                case List(Function(params, body)) =>
                  val typeDefs = params.collect {
                    case param @ Ident(name) => makeKindProjectorTypeDef(name.toTypeName).withSpan(param.span)
                  }
                  if (typeDefs.length != params.length) fail()
                  else LambdaTypeTree(typeDefs, body)
                case _ =>
                  fail()
              }
            case _ =>
              val (newArgs, tparams) = replaceKindProjectorPlaceholders(args)

              lambdaAbstract(tparams, AppliedTypeTree(applied, newArgs))
          }

        } else {
          AppliedTypeTree(applied, args)
        }
      })
      case _ =>
        if (!ctx.settings.YkindProjector.isDefault) {
          t match {
            case Tuple(params) =>
              val (newParams, tparams) = replaceKindProjectorPlaceholders(params)

              if (tparams.isEmpty) {
                t
              } else {
                LambdaTypeTree(tparams, Tuple(newParams))
              }
            case _ => t
          }
        } else {
          t
        }
    }

    private def typeProjection(t: Tree): Tree = {
      accept(HASH)
      val id = typeIdent()
      atSpan(startOffset(t), startOffset(id)) { Select(t, id.name) }
    }

    /**   ArgTypes          ::=  Type {`,' Type}
     *                        |  NamedTypeArg {`,' NamedTypeArg}
     *    NamedTypeArg      ::=  id `=' Type
     */
    def argTypes(namedOK: Boolean, wildOK: Boolean): List[Tree] = {

      def argType() = {
        val t = typ()
        if (wildOK) t else rejectWildcardType(t)
      }

      def namedTypeArg() = {
        val name = ident()
        accept(EQUALS)
        NamedArg(name.toTypeName, argType())
      }

      if (namedOK && in.token == IDENTIFIER)
        in.currentRegion.withCommasExpected {
          argType() match {
            case Ident(name) if in.token == EQUALS =>
              in.nextToken()
              commaSeparatedRest(NamedArg(name, argType()), () => namedTypeArg())
            case firstArg =>
              commaSeparatedRest(firstArg, () => argType())
          }
        }
      else commaSeparated(() => argType())
    }

    def paramTypeOf(core: () => Tree): Tree =
      if in.token == ARROW || isPureArrow(nme.PUREARROW) then
        val isImpure = in.token == ARROW
        atSpan(in.skipToken()):
          val tp = if isImpure then core() else capturesAndResult(core)
          if isImpure && Feature.pureFunsEnabled then ImpureByNameTypeTree(tp)
          else ByNameTypeTree(tp)
      else
        core()

    private def maybeInto(tp: () => Tree) =
      if in.isIdent(nme.into)
          && in.featureEnabled(Feature.into)
          && canStartTypeTokens.contains(in.lookahead.token)
      then atSpan(in.skipToken()) { Into(tp()) }
      else tp()

    /** FunArgType ::=  Type
     *               |  `=>' Type
     *               |  `->' [CaptureSet] Type
     */
    val funArgType: () => Tree = () => paramTypeOf(typ)

    /** ParamType  ::=  ParamValueType
     *               |  `=>' ParamValueType
     *               |  `->' [CaptureSet] ParamValueType
     */
    def paramType(): Tree = paramTypeOf(paramValueType)

    /** ParamValueType ::= Type [`*']
     */
    def paramValueType(): Tree = {
      val t = maybeInto(toplevelTyp)
      if (isIdent(nme.raw.STAR)) {
        in.nextToken()
        atSpan(startOffset(t)) { PostfixOp(t, Ident(tpnme.raw.STAR)) }
      }
      else t
    }

    /** TypeArgs      ::= `[' Type {`,' Type} `]'
     *  NamedTypeArgs ::= `[' NamedTypeArg {`,' NamedTypeArg} `]'
     */
    def typeArgs(namedOK: Boolean, wildOK: Boolean): List[Tree] =
      inBracketsWithCommas(argTypes(namedOK, wildOK))

    /** Refinement ::= `{' RefineStatSeq `}'
     */
    def refinement(indentOK: Boolean): List[Tree] =
      if indentOK then
        inBracesOrIndented(refineStatSeq(), rewriteWithColon = true)
      else
        inBraces(refineStatSeq())

    /** TypeBounds ::= [`>:' Type] [`<:' Type]
     */
    def typeBounds(): TypeBoundsTree =
      atSpan(in.offset) { TypeBoundsTree(bound(SUPERTYPE), bound(SUBTYPE)) }

    private def bound(tok: Int): Tree =
      if (in.token == tok) { in.nextToken(); toplevelTyp() }
      else EmptyTree

    /** TypeParamBounds   ::=  TypeBounds {`<%' Type} {`:' Type}
     */
    def typeParamBounds(pname: TypeName): Tree = {
      val t = typeBounds()
      val cbs = contextBounds(pname)
      if (cbs.isEmpty) t
      else atSpan((t.span union cbs.head.span).start) { ContextBounds(t, cbs) }
    }

    def contextBounds(pname: TypeName): List[Tree] =
      if in.isColon then
        atSpan(in.skipToken()) {
          AppliedTypeTree(toplevelTyp(), Ident(pname))
        } :: contextBounds(pname)
      else if in.token == VIEWBOUND then
        report.errorOrMigrationWarning(
          em"view bounds `<%' are no longer supported, use a context bound `:' instead",
          in.sourcePos(), from = `3.0`)
        atSpan(in.skipToken()) {
          Function(Ident(pname) :: Nil, toplevelTyp())
        } :: contextBounds(pname)
      else
        Nil

    def typedOpt(): Tree =
      if in.isColon then { in.nextToken(); toplevelTyp() }
      else TypeTree().withSpan(Span(in.lastOffset))

    def typeDependingOn(location: Location): Tree =
      if location.inParens then typ()
      else if location.inPattern then rejectWildcardType(refinedType())
      else infixType()

/* ----------- EXPRESSIONS ------------------------------------------------ */

    /** Does the current conditional expression continue after
     *  the initially parsed (...) region?
     */
    def toBeContinued(altToken: Token): Boolean =
      inline def canContinue =
        !in.canStartStatTokens.contains(in.token)  // not statement, so take as continued expr
      || followedByToken(altToken)                 // scan ahead to see whether we find a `then` or `do`

      !in.isNewLine       // a newline token means the expression is finished
      && !migrateTo3      // old syntax
      && canContinue
    end toBeContinued

    def condExpr(altToken: Token): Tree =
      val t: Tree =
        if in.token == LPAREN then
          var t: Tree = atSpan(in.offset) { Parens(inParens(exprInParens())) }
          if in.token != altToken then
            if toBeContinued(altToken) then
              t = inSepRegion(InCond) {
                expr1Rest(
                  postfixExprRest(
                    simpleExprRest(t, Location.ElseWhere),
                    Location.ElseWhere),
                  Location.ElseWhere)
              }
            else
              if rewriteToNewSyntax(t.span) then
                dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
              in.observeIndented()
              return t
          t
        else if in.isNestedStart then
          try expr() finally newLinesOpt()
        else
          inSepRegion(InCond)(expr())
      if rewriteToOldSyntax(t.span.startPos) then revertToParens(t)
      accept(altToken)
      t

    /** Expr              ::=  [`implicit'] FunParams (‘=>’ | ‘?=>’) Expr
     *                      |  HkTypeParamClause ‘=>’ Expr
     *                      |  Expr1
     *  FunParams         ::=  Bindings
     *                      |  id
     *                      |  `_'
     *  ExprInParens      ::=  PostfixExpr `:' Type
     *                      |  Expr
     *  BlockResult       ::=  [‘implicit’] FunParams (‘=>’ | ‘?=>’) Block
     *                      |  HkTypeParamClause ‘=>’ Block
     *                      |  Expr1
     *  Expr1             ::=  [‘inline’] `if' `(' Expr `)' {nl} Expr [[semi] else Expr]
     *                      |  [‘inline’] `if' Expr `then' Expr [[semi] else Expr]
     *                      |  `while' `(' Expr `)' {nl} Expr
     *                      |  `while' Expr `do' Expr
     *                      |  `try' Expr Catches [`finally' Expr]
     *                      |  `try' Expr [`finally' Expr]
     *                      |  `throw' Expr
     *                      |  `return' [Expr]
     *                      |  ForExpr
     *                      |  [SimpleExpr `.'] id `=' Expr
     *                      |  PrefixOperator SimpleExpr `=' Expr
     *                      |  SimpleExpr1 ArgumentExprs `=' Expr
     *                      |  PostfixExpr [Ascription]
     *                      |  ‘inline’ InfixExpr MatchClause
     *  Bindings          ::=  `(' [Binding {`,' Binding}] `)'
     *  Binding           ::=  (id | `_') [`:' Type]
     *  Ascription        ::=  `:' InfixType
     *                      |  `:' Annotation {Annotation}
     *                      |  `:' `_' `*'
     *  Catches           ::=  ‘catch’ (Expr | ExprCaseClause)
     */
    val exprInParens: () => Tree = () => expr(Location.InParens)

    val expr: () => Tree = () => expr(Location.ElseWhere)

    def subExpr() = subPart(expr)

    def expr(location: Location): Tree = {
      val start = in.offset
      in.token match
        case IMPLICIT =>
          closure(start, location, modifiers(BitSet(IMPLICIT)))
        case LBRACKET =>
          val start = in.offset
          val tparams = typeParamClause(ParamOwner.TypeParam)
          val arrowOffset = accept(ARROW)
          val body = expr(location)
          atSpan(start, arrowOffset) {
            getFunction(body) match
              case Some(f) =>
                checkFunctionNotErased(f, "poly function")
                PolyFunction(tparams, f)
              case None =>
                syntaxError(em"Implementation restriction: polymorphic function literals must have a value parameter", arrowOffset)
                errorTermTree(arrowOffset)
          }
        case _ =>
          val saved = placeholderParams
          placeholderParams = Nil

          def wrapPlaceholders(t: Tree) = try
            if (placeholderParams.isEmpty) t
            else new WildcardFunction(placeholderParams.reverse, t)
          finally placeholderParams = saved

          val t = expr1(location)
          if in.isArrow then
            placeholderParams = Nil // don't interpret `_' to the left of `=>` as placeholder
            wrapPlaceholders(closureRest(start, location, convertToParams(t)))
          else if isWildcard(t) then
            placeholderParams = placeholderParams ::: saved
            t
          else
            checkNonParamTuple(t)
            wrapPlaceholders(t)
    }

    def expr1(location: Location = Location.ElseWhere): Tree = in.token match
      case IF =>
        ifExpr(in.offset, If)
      case WHILE =>
        atSpan(in.skipToken()) {
          val cond = condExpr(DO)
          newLinesOpt()
          val body = subExpr()
          WhileDo(cond, body)
        }
      case DO =>
        report.errorOrMigrationWarning(
          em"""`do <body> while <cond>` is no longer supported,
              |use `while <body> ; <cond> do ()` instead.${rewriteNotice()}""",
          in.sourcePos(), from = `3.0`)
        val start = in.skipToken()
        atSpan(start) {
          val body = expr()
          if (isStatSep) in.nextToken()
          val whileStart = in.offset
          accept(WHILE)
          val cond = expr()
          if sourceVersion.isMigrating then
            patch(source, Span(start, start + 2), "while ({")
            patch(source, Span(whileStart, whileStart + 5), ";")
            cond match {
              case Parens(_) =>
                patch(source, Span(cond.span.start, cond.span.start + 1), "")
                patch(source, Span(cond.span.end - 1, cond.span.end), "")
              case _ =>
            }
            patch(source, cond.span.endPos, "}) ()")
          WhileDo(Block(body, cond), unitLiteral)
        }
      case TRY =>
        val tryOffset = in.offset
        atSpan(in.skipToken()) {
          val body = expr()
          val (handler, handlerStart) =
            if in.token == CATCH then
              val span = in.offset
              in.nextToken()
              (if in.token == CASE then Match(EmptyTree, caseClause(exprOnly = true) :: Nil)
                else subExpr(),
                span)
            else (EmptyTree, -1)

          handler match {
            case Block(Nil, EmptyTree) =>
              assert(handlerStart != -1)
              syntaxErrorOrIncomplete(
                EmptyCatchBlock(body),
                Span(handlerStart, endOffset(handler))
              )
            case _ =>
          }

          val finalizer =
            if (in.token == FINALLY) {
              in.nextToken();
              val expr = subExpr()
              if expr.span.exists then expr
              else unitLiteral // finally without an expression
            }
            else {
              if handler.isEmpty then
                report.warning(
                  EmptyCatchAndFinallyBlock(body),
                  source.atSpan(Span(tryOffset, endOffset(body)))
                )
              EmptyTree
            }
          ParsedTry(body, handler, finalizer)
        }
      case THROW =>
        atSpan(in.skipToken()) { Throw(expr()) }
      case RETURN =>
        atSpan(in.skipToken()) {
          Return(if (isExprIntro) expr() else EmptyTree, EmptyTree)
        }
      case FOR =>
        forExpr()
      case _ =>
        if isIdent(nme.inline)
           && !in.inModifierPosition()
           && in.canStartExprTokens.contains(in.lookahead.token)
        then
          val start = in.skipToken()
          in.token match
            case IF =>
              ifExpr(start, InlineIf)
            case _ =>
              postfixExpr() match
                case t @ Match(scrut, cases) =>
                  InlineMatch(scrut, cases).withSpan(t.span)
                case t =>
                  syntaxError(em"`inline` must be followed by an `if` or a `match`", start)
                  t
        else expr1Rest(postfixExpr(location), location)
    end expr1

    def expr1Rest(t: Tree, location: Location): Tree =
      if in.token == EQUALS then
        t match
          case Ident(_) | Select(_, _) | Apply(_, _) | PrefixOp(_, _) =>
            atSpan(startOffset(t), in.skipToken()) {
              val loc = if location.inArgs then location else Location.ElseWhere
              Assign(t, subPart(() => expr(loc)))
            }
          case _ =>
            t
      else if in.isColon then
        in.nextToken()
        ascription(t, location)
      else
        t

    def ascription(t: Tree, location: Location): Tree = atSpan(startOffset(t)) {
      in.token match {
        case USCORE if in.lookahead.isIdent(nme.raw.STAR) =>
          val uscoreStart = in.skipToken()
          val isVarargSplice = location.inArgs && followingIsVararg()
          in.nextToken()
          if isVarargSplice then
            report.errorOrMigrationWarning(
              em"The syntax `x: _*` is no longer supported for vararg splices; use `x*` instead${rewriteNotice(`future-migration`)}",
              in.sourcePos(uscoreStart),
              future)
            if sourceVersion == `future-migration` then
              patch(source, Span(t.span.end, in.lastOffset), " *")
          else if opStack.nonEmpty then
            report.errorOrMigrationWarning(
              em"""`_*` can be used only for last argument of method application.
                  |It is no longer allowed in operands of infix operations.""",
              in.sourcePos(uscoreStart), from = `3.0`)
          else
            syntaxError(SeqWildcardPatternPos(), uscoreStart)
          Typed(t, atSpan(uscoreStart) { Ident(tpnme.WILDCARD_STAR) })
        case AT if !location.inPattern =>
          annotations().foldLeft(t)(Annotated)
        case _ =>
          val tpt = typeDependingOn(location)
          if (isWildcard(t) && !location.inPattern) {
            val vd :: rest = placeholderParams: @unchecked
            placeholderParams =
              cpy.ValDef(vd)(tpt = tpt).withSpan(vd.span.union(tpt.span)) :: rest
          }
          Typed(t, tpt)
      }
    }

    /**    `if' `(' Expr `)' {nl} Expr [[semi] else Expr]
     *     `if' Expr `then' Expr [[semi] else Expr]
     */
    def ifExpr(start: Offset, mkIf: (Tree, Tree, Tree) => If): If =
      atSpan(start, in.skipToken()) {
        val cond = condExpr(THEN)
        newLinesOpt()
        val thenp = subExpr()
        val elsep = if (in.token == ELSE) { in.nextToken(); subExpr() }
                    else EmptyTree
        mkIf(cond, thenp, elsep)
      }

    /**    MatchClause ::= `match' `{' CaseClauses `}'
     */
    def matchClause(t: Tree): Match =
      atSpan(startOffset(t), in.skipToken()) {
        Match(t, inBracesOrIndented(caseClauses(() => caseClause())))
      }

    /**    `match' <<< TypeCaseClauses >>>
     */
    def matchType(t: Tree): MatchTypeTree =
      atSpan(startOffset(t), accept(MATCH)) {
        MatchTypeTree(EmptyTree, t, inBracesOrIndented(caseClauses(typeCaseClause)))
      }

    /** FunParams         ::=  Bindings
     *                     |   id
     *                     |   `_'
     *  Bindings          ::=  `(' [Binding {`,' Binding}] `)'
     */
    def funParams(mods: Modifiers, location: Location): List[Tree] =
      if in.token == LPAREN then
        in.nextToken()
        if in.token == RPAREN then
          Nil
        else
          try
            commaSeparated(() => binding(mods))
          finally
            accept(RPAREN)
      else {
        val start = in.offset
        val name = bindingName()
        val t =
          if ((in.token == COLONop || in.token == COLONfollow) && location == Location.InBlock) {
            report.errorOrMigrationWarning(
              em"This syntax is no longer supported; parameter needs to be enclosed in (...)${rewriteNotice(`future-migration`)}",
              source.atSpan(Span(start, in.lastOffset)),
              from = future)
            in.nextToken()
            val t = infixType()
            if (sourceVersion == `future-migration`) {
              patch(source, Span(start), "(")
              patch(source, Span(in.lastOffset), ")")
            }
            t
          }
          else TypeTree()
        (atSpan(start) { makeParameter(name, t, mods) }) :: Nil
      }

    /**  Binding           ::= [`erased`] (id | `_') [`:' Type]
     */
    def binding(mods: Modifiers): Tree =
      atSpan(in.offset) {
        val mods1 = if isErasedKw then addModifier(mods) else mods
        makeParameter(bindingName(), typedOpt(), mods1)
      }

    def bindingName(): TermName =
      if (in.token == USCORE) {
        in.nextToken()
        WildcardParamName.fresh()
      }
      else ident()

    /** Expr         ::= [‘implicit’] FunParams `=>' Expr
     *  BlockResult  ::= implicit id [`:' InfixType] `=>' Block // Scala2 only
     */
    def closure(start: Int, location: Location, implicitMods: Modifiers): Tree =
      closureRest(start, location, funParams(implicitMods, location))

    def closureRest(start: Int, location: Location, params: List[Tree]): Tree =
      atSpan(start, in.offset) {
        if in.token == CTXARROW then
          if params.isEmpty then
            syntaxError(em"context function literals require at least one formal parameter", Span(start, in.lastOffset))
          in.nextToken()
        else
          accept(ARROW)
        val body =
          if location == Location.InBlock then block()
          else if location == Location.InColonArg && in.token == INDENT then blockExpr()
          else expr()
        Function(params, body)
      }

    /** PostfixExpr   ::= InfixExpr [id [nl]]
     *  InfixExpr     ::= PrefixExpr
     *                  | InfixExpr id [nl] InfixExpr
     *                  | InfixExpr id ColonArgument
     *                  | InfixExpr MatchClause
     */
    def postfixExpr(location: Location = Location.ElseWhere): Tree =
      val t = postfixExprRest(prefixExpr(location), location)
      if location.inArgs && followingIsVararg() then
        Typed(t, atSpan(in.skipToken()) { Ident(tpnme.WILDCARD_STAR) })
      else
        t

    def postfixExprRest(t: Tree, location: Location): Tree =
      infixOps(t, in.canStartExprTokens, prefixExpr, location, ParseKind.Expr,
        isOperator = !(location.inArgs && followingIsVararg())
                     && nextCanFollowOperator(canStartInfixExprTokens))

    /** PrefixExpr       ::= [PrefixOperator'] SimpleExpr
     *  PrefixOperator   ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’ (if not backquoted)
     */
    val prefixExpr: Location => Tree = location =>
      if in.token == IDENTIFIER && nme.raw.isUnary(in.name)
         && in.canStartExprTokens.contains(in.lookahead.token)
      then
        val start = in.offset
        val op = termIdent()
        if (op.name == nme.raw.MINUS && isNumericLit)
          simpleExprRest(literal(start), location, canApply = true)
        else
          atSpan(start) { PrefixOp(op, simpleExpr(location)) }
      else simpleExpr(location)

    /** SimpleExpr    ::= ‘new’ ConstrApp {`with` ConstrApp} [TemplateBody]
     *                 |  ‘new’ TemplateBody
     *                 |  BlockExpr
     *                 |  ExprSplice
     *                 |  Quoted
     *                 |  quoteId
     *                 |  SimpleExpr1 [`_`]
     *  SimpleExpr1   ::= literal
     *                 |  xmlLiteral
     *                 |  SimpleRef
     *                 |  `(` [ExprsInParens] `)`
     *                 |  SimpleExpr `.` id
     *                 |  SimpleExpr `.` MatchClause
     *                 |  SimpleExpr (TypeArgs | NamedTypeArgs)
     *                 |  SimpleExpr1 ArgumentExprs
     *                 |  SimpleExpr1 ColonArgument
     *  ColonArgument ::= colon [LambdaStart]
     *                    indent (CaseClauses | Block) outdent
     *  LambdaStart   ::= FunParams (‘=>’ | ‘?=>’)
     *                 |  HkTypeParamClause ‘=>’
     *  ColonArgBody  ::= indent (CaseClauses | Block) outdent
     *  Quoted        ::= ‘'’ ‘{’ Block ‘}’
     *                 |  ‘'’ ‘[’ Type ‘]’
     */
    def simpleExpr(location: Location): Tree = {
      var canApply = true
      val t = in.token match {
        case XMLSTART =>
          xmlLiteral()
        case IDENTIFIER =>
          if (isSplice) splice(isType = false)
          else simpleRef()
        case BACKQUOTED_IDENT | THIS | SUPER =>
          simpleRef()
        case USCORE =>
          val start = in.skipToken()
          val pname = WildcardParamName.fresh()
          val param = ValDef(pname, TypeTree(), EmptyTree).withFlags(SyntheticTermParam)
            .withSpan(Span(start))
          placeholderParams = param :: placeholderParams
          atSpan(start) { Ident(pname) }
        case LPAREN =>
          atSpan(in.offset) { makeTupleOrParens(inParensWithCommas(exprsInParensOrBindings())) }
        case LBRACE | INDENT =>
          canApply = false
          blockExpr()
        case QUOTE =>
          atSpan(in.skipToken()) {
            withinStaged(StageKind.Quoted | (if (location.inPattern) StageKind.QuotedPattern else 0)) {
              val body =
                if (in.token == LBRACKET) inBrackets(typ())
                else stagedBlock()
              Quote(body, Nil)
            }
          }
        case NEW =>
          canApply = false
          newExpr()
        case MACRO =>
          val start = in.skipToken()
          MacroTree(simpleExpr(Location.ElseWhere))
        case _ =>
          if isLiteral then
            literal()
          else if in.isColon then
            syntaxError(IllegalStartSimpleExpr(tokenString(in.token)))
            in.nextToken()
            simpleExpr(location)
          else
            val start = in.lastOffset
            syntaxErrorOrIncomplete(IllegalStartSimpleExpr(tokenString(in.token)), expectedOffset)
            errorTermTree(start)
      }
      simpleExprRest(t, location, canApply)
    }

    def simpleExprRest(t: Tree, location: Location, canApply: Boolean = true): Tree =
      if (canApply) argumentStart()
      in.token match
        case DOT =>
          in.nextToken()
          simpleExprRest(selectorOrMatch(t), location, canApply = true)
        case LBRACKET =>
          val tapp = atSpan(startOffset(t), in.offset) { TypeApply(t, typeArgs(namedOK = true, wildOK = false)) }
          simpleExprRest(tapp, location, canApply = true)
        case LPAREN | LBRACE | INDENT if canApply =>
          val app = atSpan(startOffset(t), in.offset) { mkApply(t, argumentExprs()) }
          simpleExprRest(app, location, canApply = true)
        case USCORE =>
          atSpan(startOffset(t), in.skipToken()) { PostfixOp(t, Ident(nme.WILDCARD)) }
        case _ =>
          if in.isColon && location == Location.InParens && followingIsLambdaParams() then
            t match
              case id @ Ident(name) =>
                if name.is(WildcardParamName) then
                  assert(name == placeholderParams.head.name)
                  placeholderParams = placeholderParams.tail
                atSpan(startOffset(id)) {
                  makeParameter(name.asTermName, typedOpt(), Modifiers(), isBackquoted = isBackquoted(id))
                }
              case _ => t
          else if isColonLambda then
            val app = atSpan(startOffset(t), in.skipToken()) {
              Apply(t, expr(Location.InColonArg) :: Nil)
            }
            simpleExprRest(app, location, canApply = true)
          else t
    end simpleExprRest

    /** SimpleExpr    ::=  ‘new’ ConstrApp {`with` ConstrApp} [TemplateBody]
     *                  |  ‘new’ TemplateBody
     */
    def newExpr(): Tree =
      val start = in.skipToken()
      def reposition(t: Tree) = t.withSpan(Span(start, in.lastOffset))
      possibleTemplateStart()
      val parents =
        if in.isNestedStart then Nil
        else constrApps(exclude = COMMA)
      possibleTemplateStart(isNew = true)
      parents match {
        case parent :: Nil if !in.isNestedStart =>
          reposition(if (parent.isType) ensureApplied(wrapNew(parent)) else parent)
        case tkn if in.token == INDENT =>
          New(templateBodyOpt(emptyConstructor, parents, Nil))
        case _ =>
          New(reposition(templateBodyOpt(emptyConstructor, parents, Nil)))
      }

    /**   ExprsInParens     ::=  ExprInParens {`,' ExprInParens}
     *    Bindings          ::=  Binding {`,' Binding}
     */
    def exprsInParensOrBindings(): List[Tree] =
      if in.token == RPAREN then Nil
      else in.currentRegion.withCommasExpected {
        var isFormalParams = false
        def exprOrBinding() =
          if isErasedKw then isFormalParams = true
          if isFormalParams then binding(Modifiers())
          else
            val t = exprInParens()
            if t.isInstanceOf[ValDef] then isFormalParams = true
            t
        commaSeparatedRest(exprOrBinding(), exprOrBinding)
      }

    /** ParArgumentExprs ::= `(' [‘using’] [ExprsInParens] `)'
     *                    |  `(' [ExprsInParens `,'] PostfixExpr `*' ')'
     */
    def parArgumentExprs(): (List[Tree], Boolean) =
      inParensWithCommas:
        if in.token == RPAREN then
          (Nil, false)
        else if isIdent(nme.using) then
          in.nextToken()
          (commaSeparated(argumentExpr), true)
        else
          (commaSeparated(argumentExpr), false)

    /** ArgumentExprs ::= ParArgumentExprs
     *                 |  [nl] BlockExpr
     */
    def argumentExprs(): (List[Tree], Boolean) =
      if (in.isNestedStart) (blockExpr() :: Nil, false) else parArgumentExprs()

    def mkApply(fn: Tree, args: (List[Tree], Boolean)): Tree =
      val res = Apply(fn, args._1)
      if args._2 then res.setApplyKind(ApplyKind.Using)
      res

    val argumentExpr: () => Tree = () => expr(Location.InArgs) match
      case arg @ Assign(Ident(id), rhs) => cpy.NamedArg(arg)(id, rhs)
      case arg => arg

    /** ArgumentExprss ::= {ArgumentExprs}
     */
    def argumentExprss(fn: Tree): Tree = {
      argumentStart()
      if (in.token == LPAREN || in.isNestedStart) argumentExprss(mkApply(fn, argumentExprs()))
      else fn
    }

    /** ParArgumentExprss ::= {ParArgumentExprs}
     *
     *  Special treatment for arguments to primary constructor annotations.
     *  (...) is considered an argument only if it does not look like a formal
     *  parameter list, i.e. does not start with `( <annot>* <mod>* ident : `
     *  Furthermore, `()` is considered a annotation argument only if it comes first.
     */
    def parArgumentExprss(fn: Tree): Tree = {
      def isLegalAnnotArg: Boolean = {
        val lookahead = in.LookaheadScanner()
        (lookahead.token == LPAREN) && {
          lookahead.nextToken()
          if (lookahead.token == RPAREN)
            !fn.isInstanceOf[Trees.Apply[?]] // allow one () as annotation argument
          else if lookahead.token == IDENTIFIER then
            lookahead.nextToken()
            !lookahead.isColon
          else in.canStartExprTokens.contains(lookahead.token)
        }
      }
      if (in.token == LPAREN && (!inClassConstrAnnots || isLegalAnnotArg))
        parArgumentExprss(
          atSpan(startOffset(fn)) { mkApply(fn, parArgumentExprs()) }
        )
      else fn
    }

    /** BlockExpr     ::= <<< (CaseClauses | Block) >>>
     */
    def blockExpr(): Tree = atSpan(in.offset) {
      val simplify = in.token == INDENT
      inDefScopeBraces {
        if (in.token == CASE) Match(EmptyTree, caseClauses(() => caseClause()))
        else block(simplify)
      }
    }

    /** Block ::= BlockStatSeq
     *  @note  Return tree does not have a defined span.
     */
    def block(simplify: Boolean = false): Tree = {
      val stats = blockStatSeq()
      def isExpr(stat: Tree) = !(stat.isDef || stat.isInstanceOf[Import])
      if (stats.nonEmpty && isExpr(stats.last)) {
        val inits = stats.init
        val last = stats.last
        if (inits.isEmpty && (simplify || last.isInstanceOf[Block])) last
        else Block(inits, last)
      }
      else Block(stats, EmptyTree)
    }

    /** Guard ::= if PostfixExpr
     */
    def guard(): Tree =
      if (in.token == IF) { in.nextToken(); postfixExpr(Location.InGuard) }
      else EmptyTree

    /** Enumerators ::= Generator {semi Enumerator | Guard}
     */
    def enumerators(): List[Tree] = generator() :: enumeratorsRest()

    def enumeratorsRest(): List[Tree] =
      if (isStatSep) {
        in.nextToken()
        if (in.token == DO || in.token == YIELD || in.token == RBRACE) Nil
        else enumerator() :: enumeratorsRest()
      }
      else if (in.token == IF)
        guard() :: enumeratorsRest()
      else Nil

    /** Enumerator  ::=  Generator
     *                |  Guard {Guard}
     *                |  Pattern1 `=' Expr
     */
    def enumerator(): Tree =
      if (in.token == IF) guard()
      else if (in.token == CASE) generator()
      else {
        val pat = pattern1()
        if (in.token == EQUALS) atSpan(startOffset(pat), in.skipToken()) { GenAlias(pat, subExpr()) }
        else generatorRest(pat, casePat = false)
      }

    /** Generator   ::=  [‘case’] Pattern `<-' Expr
     */
    def generator(): Tree = {
      val casePat = if (in.token == CASE) { in.nextToken(); true } else false
      generatorRest(pattern1(), casePat)
    }

    def generatorRest(pat: Tree, casePat: Boolean): GenFrom =
      atSpan(startOffset(pat), accept(LARROW)) {
        val checkMode =
          if casePat then GenCheckMode.FilterAlways
          else if sourceVersion.isAtLeast(`future`) then GenCheckMode.Check
          else if sourceVersion.isAtLeast(`3.2`) then GenCheckMode.CheckAndFilter
          else GenCheckMode.FilterNow  // filter on source version < 3.2, for backward compat
        GenFrom(pat, subExpr(), checkMode)
      }

    /** ForExpr  ::=  ‘for’ ‘(’ Enumerators ‘)’ {nl} [‘do‘ | ‘yield’] Expr
     *             |  ‘for’ ‘{’ Enumerators ‘}’ {nl} [‘do‘ | ‘yield’] Expr
     *             |  ‘for’     Enumerators          (‘do‘ | ‘yield’) Expr
     */
    def forExpr(): Tree =
      atSpan(in.skipToken()) {
        var wrappedEnums = true
        val start = in.offset
        val forEnd = in.lastOffset
        val leading = in.token
        val enums =
          if (leading == LBRACE || leading == LPAREN && followingIsEnclosedGenerators()) {
            in.nextToken()
            val res =
              if (leading == LBRACE || in.token == CASE)
                enumerators()
              else {
                val pats = patternsOpt()
                val pat =
                  if (in.token == RPAREN || pats.length > 1) {
                    wrappedEnums = false
                    accept(RPAREN)
                    atSpan(start) { makeTupleOrParens(pats) } // note: alternatives `|' need to be weeded out by typer.
                  }
                  else pats.head
                generatorRest(pat, casePat = false) :: enumeratorsRest()
              }
            if (wrappedEnums) {
              val closingOnNewLine = in.isAfterLineEnd
              accept(leading + 1)
              def hasMultiLineEnum =
                res.exists { t =>
                  val pos = t.sourcePos
                  pos.startLine < pos.endLine
                }
              if in.newSyntax && in.rewrite && (leading == LBRACE || !hasMultiLineEnum) then
                // Don't rewrite if that could change meaning of newlines
                newLinesOpt()
                dropParensOrBraces(start, if (in.token == YIELD || in.token == DO) "" else "do")
            }
            in.observeIndented()
            res
          }
          else {
            wrappedEnums = false

            if (in.token == INDENT)
              inBracesOrIndented(enumerators())
            else {
              val ts = inSepRegion(InFor)(enumerators())
              if (rewriteToOldSyntax(Span(start)) && ts.nonEmpty)
                if (ts.head.sourcePos.startLine != ts.last.sourcePos.startLine) {
                  patch(source, Span(forEnd), " {")
                  patch(source, Span(in.offset), "} ")
                }
                else {
                  patch(source, ts.head.span.startPos, "(")
                  patch(source, ts.last.span.endPos, ")")
                }
              ts
            }
          }
        newLinesOpt()
        if (in.token == YIELD) {
          in.nextToken()
          ForYield(enums, subExpr())
        }
        else if (in.token == DO) {
          if (rewriteToOldSyntax()) dropTerminator()
          in.nextToken()
          ForDo(enums, subExpr())
        }
        else {
          if (!wrappedEnums) syntaxErrorOrIncomplete(YieldOrDoExpectedInForComprehension())
          ForDo(enums, expr())
        }
      }

    /** CaseClauses         ::= CaseClause {CaseClause}
     *  TypeCaseClauses     ::= TypeCaseClause {TypeCaseClause}
     */
    def caseClauses(clause: () => CaseDef): List[CaseDef] = {
      val buf = new ListBuffer[CaseDef]
      buf += clause()
      while (in.token == CASE) buf += clause()
      buf.toList
    }

    /** CaseClause         ::= ‘case’ Pattern [Guard] `=>' Block
     *  ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
     */
    def caseClause(exprOnly: Boolean = false): CaseDef = atSpan(in.offset) {
      val (pat, grd) = inSepRegion(InCase) {
        accept(CASE)
        (pattern(), guard())
      }
      CaseDef(pat, grd, atSpan(accept(ARROW)) {
        if exprOnly then
          if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
            warning(em"""Misleading indentation: this expression forms part of the preceding catch case.
                        |If this is intended, it should be indented for clarity.
                        |Otherwise, if the handler is intended to be empty, use a multi-line catch with
                        |an indented case.""")
          expr()
        else block()
      })
    }

    /** TypeCaseClause     ::= ‘case’ (InfixType | ‘_’) ‘=>’ Type [semi]
     */
    def typeCaseClause(): CaseDef = atSpan(in.offset) {
      val pat = inSepRegion(InCase) {
        accept(CASE)
        in.token match {
          case USCORE if in.lookahead.isArrow =>
            val start = in.skipToken()
            Ident(tpnme.WILDCARD).withSpan(Span(start, in.lastOffset, start))
          case _ =>
            rejectWildcardType(infixType())
        }
      }
      CaseDef(pat, EmptyTree, atSpan(accept(ARROW)) {
        val t = rejectWildcardType(typ())
        if in.token == SEMI then in.nextToken()
        newLinesOptWhenFollowedBy(CASE)
        t
      })
    }

    /* -------- PATTERNS ------------------------------------------- */

    /**  Pattern           ::=  Pattern1 { `|' Pattern1 }
     */
    def pattern(location: Location = Location.InPattern): Tree =
      val pat = pattern1(location)
      if (isIdent(nme.raw.BAR))
        atSpan(startOffset(pat)) { Alternative(pat :: patternAlts(location)) }
      else pat

    def patternAlts(location: Location): List[Tree] =
      if (isIdent(nme.raw.BAR)) { in.nextToken(); pattern1(location) :: patternAlts(location) }
      else Nil

    /**  Pattern1     ::= PatVar Ascription
     *                  | [‘-’] integerLiteral Ascription
     *                  | [‘-’] floatingPointLiteral Ascription
     *                  | Pattern2
     */
    def pattern1(location: Location = Location.InPattern): Tree =
      val p = pattern2()
      if in.isColon then
        val isVariableOrNumber = isVarPattern(p) || p.isInstanceOf[Number]
        if !isVariableOrNumber then
          report.gradualErrorOrMigrationWarning(
            em"""Type ascriptions after patterns other than:
                |  * variable pattern, e.g. `case x: String =>`
                |  * number literal pattern, e.g. `case 10.5: Double =>`
                |are no longer supported. Remove the type ascription or move it to a separate variable pattern.""",
            in.sourcePos(),
            warnFrom = `3.3`,
            errorFrom = future
          )
        in.nextToken()
        ascription(p, location)
      else p

    /**  Pattern3    ::=  InfixPattern
     *                 |  PatVar ‘*’
     */
    def pattern3(): Tree =
      val p = infixPattern()
      if followingIsVararg() then
        val start = in.skipToken()
        p match
          case p @ Ident(name) if name.isVarPattern =>
            Typed(p, atSpan(start) { Ident(tpnme.WILDCARD_STAR) })
          case _ =>
            syntaxError(em"`*` must follow pattern variable", start)
            p
      else p

    /**  Pattern2    ::=  [id `@'] Pattern3
     */
    val pattern2: () => Tree = () => pattern3() match
      case p @ Ident(name) if in.token == AT =>
        val offset = in.skipToken()
        pattern3() match {
          case pt @ Bind(nme.WILDCARD, pt1: Typed) if pt.mods.is(Given) =>
            atSpan(startOffset(p), 0) { Bind(name, pt1).withMods(pt.mods) }
          case Typed(Ident(nme.WILDCARD), pt @ Ident(tpnme.WILDCARD_STAR)) =>
            atSpan(startOffset(p), 0) { Typed(p, pt) }
          case pt =>
            atSpan(startOffset(p), 0) { Bind(name, pt) }
        }
      case p =>
        p

    private def warnStarMigration(p: Tree) =
      report.errorOrMigrationWarning(
        em"The syntax `x: _*` is no longer supported for vararg splices; use `x*` instead",
        in.sourcePos(startOffset(p)),
        from = future)

    /**  InfixPattern ::= SimplePattern {id [nl] SimplePattern}
     */
    def infixPattern(): Tree =
      infixOps(
        simplePattern(), in.canStartExprTokens, simplePatternFn, Location.InPattern, ParseKind.Pattern,
        isOperator = in.name != nme.raw.BAR && !followingIsVararg()
                     && nextCanFollowOperator(canStartPatternTokens))

    /** SimplePattern    ::= PatVar
     *                    |  Literal
     *                    |  Quoted
     *                    |  XmlPattern
     *                    |  `(' [Patterns] `)'
     *                    |  SimplePattern1 [TypeArgs] [ArgumentPatterns]
     *  SimplePattern1   ::= SimpleRef
     *                    |  SimplePattern1 `.' id
     *  PatVar           ::= id
     *                    |  `_'
     */
    def simplePattern(): Tree = in.token match {
      case IDENTIFIER | BACKQUOTED_IDENT | THIS | SUPER =>
        simpleRef() match
          case id @ Ident(nme.raw.MINUS) if isNumericLit => literal(startOffset(id))
          case t => simplePatternRest(t)
      case USCORE =>
        wildcardIdent()
      case LPAREN =>
        atSpan(in.offset) { makeTupleOrParens(inParensWithCommas(patternsOpt())) }
      case QUOTE =>
        simpleExpr(Location.InPattern)
      case XMLSTART =>
        xmlLiteralPattern()
      case GIVEN =>
        atSpan(in.offset) {
          val givenMod = atSpan(in.skipToken())(Mod.Given())
          val typed = Typed(Ident(nme.WILDCARD), refinedType())
          Bind(nme.WILDCARD, typed).withMods(addMod(Modifiers(), givenMod))
        }
      case _ =>
        if (isLiteral) literal(inPattern = true)
        else {
          val start = in.lastOffset
          syntaxErrorOrIncomplete(IllegalStartOfSimplePattern(), expectedOffset)
          errorTermTree(start)
        }
    }

    val simplePatternFn: Location => Tree = _ => simplePattern()

    def simplePatternRest(t: Tree): Tree =
      if in.token == DOT then
        in.nextToken()
        simplePatternRest(selector(t))
      else
        var p = t
        if (in.token == LBRACKET)
          p = atSpan(startOffset(t), in.offset) { TypeApply(p, typeArgs(namedOK = false, wildOK = false)) }
        if (in.token == LPAREN)
          p = atSpan(startOffset(t), in.offset) { Apply(p, argumentPatterns()) }
        p

    /** Patterns          ::=  Pattern [`,' Pattern]
     */
    def patterns(location: Location = Location.InPattern): List[Tree] =
      commaSeparated(() => pattern(location))

    def patternsOpt(location: Location = Location.InPattern): List[Tree] =
      if (in.token == RPAREN) Nil else patterns(location)

    /** ArgumentPatterns  ::=  ‘(’ [Patterns] ‘)’
     *                      |  ‘(’ [Patterns ‘,’] PatVar ‘*’ ‘)’
     */
    def argumentPatterns(): List[Tree] =
      inParensWithCommas(patternsOpt(Location.InPatternArgs))

/* -------- MODIFIERS and ANNOTATIONS ------------------------------------------- */

    private def modOfToken(tok: Int, name: Name): Mod = tok match {
      case ABSTRACT    => Mod.Abstract()
      case FINAL       => Mod.Final()
      case IMPLICIT    => Mod.Implicit()
      case GIVEN       => Mod.Given()
      case LAZY        => Mod.Lazy()
      case OVERRIDE    => Mod.Override()
      case PRIVATE     => Mod.Private()
      case PROTECTED   => Mod.Protected()
      case SEALED      => Mod.Sealed()
      case IDENTIFIER =>
        name match {
          case nme.erased if in.erasedEnabled => Mod.Erased()
          case nme.inline => Mod.Inline()
          case nme.opaque => Mod.Opaque()
          case nme.open => Mod.Open()
          case nme.transparent => Mod.Transparent()
          case nme.infix => Mod.Infix()
        }
    }

    /** Drop `private' modifier when followed by a qualifier.
     *  Contract `abstract' and `override' to ABSOVERRIDE
     */
    private def normalize(mods: Modifiers): Modifiers =
      if (mods.is(Private) && mods.hasPrivateWithin)
        normalize(mods &~ Private)
      else if (mods.isAllOf(AbstractOverride))
        normalize(addFlag(mods &~ AbstractOverride, AbsOverride))
      else
        mods

    private def addModifier(mods: Modifiers): Modifiers = {
      val tok = in.token
      val name = in.name
      val mod = atSpan(in.skipToken()) { modOfToken(tok, name) }

      if mods.isOneOf(mod.flags) then
        syntaxError(RepeatedModifier(mod.flags.flagsString, source, mod.span), mod.span)
      addMod(mods, mod)
    }

    def addFlag(mods: Modifiers, flag: FlagSet): Modifiers =
      mods.withAddedFlags(flag, Span(in.offset))

    /** Always add the syntactic `mod`, but check and conditionally add semantic `mod.flags`
     */
    def addMod(mods: Modifiers, mod: Mod): Modifiers =
      addFlag(mods, mod.flags).withAddedMod(mod)

    /** AccessQualifier ::= "[" (id | this) "]"
     */
    def accessQualifierOpt(mods: Modifiers): Modifiers =
      if (in.token == LBRACKET) {
        if (mods.is(Local) || mods.hasPrivateWithin)
          syntaxError(DuplicatePrivateProtectedQualifier())
        inBrackets {
          if in.token == THIS then
            if sourceVersion.isAtLeast(future) then
              deprecationWarning(
                em"The [this] qualifier will be deprecated in the future; it should be dropped.")
            in.nextToken()
            mods | Local
          else mods.withPrivateWithin(ident().toTypeName)
        }
      }
      else mods

    /** {Annotation} {Modifier}
     *  Modifiers      ::= {Modifier}
     *  LocalModifiers ::= {LocalModifier}
     *  AccessModifier ::= (private | protected) [AccessQualifier]
     *  Modifier       ::= LocalModifier
     *                  |  AccessModifier
     *                  |  override
     *                  |  opaque
     *  LocalModifier  ::= abstract | final | sealed | open | implicit | lazy | inline | transparent | infix | erased
     */
    def modifiers(allowed: BitSet = modifierTokens, start: Modifiers = Modifiers()): Modifiers = {
      @tailrec
      def loop(mods: Modifiers): Modifiers =
        if allowed.contains(in.token)
           || in.isSoftModifier
              && localModifierTokens.subsetOf(allowed) // soft modifiers are admissible everywhere local modifiers are
              && !in.lookahead.isColon
        then
          val isAccessMod = accessModifierTokens contains in.token
          val mods1 = addModifier(mods)
          loop(if (isAccessMod) accessQualifierOpt(mods1) else mods1)
        else if (in.isNewLine && (mods.hasFlags || mods.hasAnnotations)) {
          in.nextToken()
          loop(mods)
        }
        else
          mods
      normalize(loop(start))
    }

    /** Wrap annotation or constructor in New(...).<init> */
    def wrapNew(tpt: Tree): Select = Select(New(tpt), nme.CONSTRUCTOR)

    /** Adjust start of annotation or constructor to offset of preceding @ or new */
    def adjustStart(start: Offset)(tree: Tree): Tree = {
      val tree1 = tree match {
        case Apply(fn, args) => cpy.Apply(tree)(adjustStart(start)(fn), args)
        case Select(qual, name) => cpy.Select(tree)(adjustStart(start)(qual), name)
        case _ => tree
      }
      if (tree1.span.exists && start < tree1.span.start)
        tree1.withSpan(tree1.span.withStart(start))
      else tree1
    }

    /** Annotation        ::=  `@' SimpleType1 {ParArgumentExprs}
     */
    def annot(): Tree =
      adjustStart(accept(AT)) {
        ensureApplied(parArgumentExprss(wrapNew(simpleType1())))
      }

    def annotations(skipNewLines: Boolean = false): List[Tree] = {
      if (skipNewLines) newLinesOptWhenFollowedBy(AT)
      if (in.token == AT) annot() :: annotations(skipNewLines)
      else Nil
    }

    def annotsAsMods(skipNewLines: Boolean = false): Modifiers =
      Modifiers() withAnnotations annotations(skipNewLines)

    def defAnnotsMods(allowed: BitSet): Modifiers =
      modifiers(allowed, annotsAsMods(skipNewLines = true))

 /* -------- PARAMETERS ------------------------------------------- */

    /** DefParamClauses       ::= DefParamClause { DefParamClause }  -- and two DefTypeParamClause cannot be adjacent
     *  DefParamClause        ::= DefTypeParamClause
     *                          | DefTermParamClause
     *                          | UsingParamClause
     */
    def typeOrTermParamClauses(
      paramOwner: ParamOwner, numLeadParams: Int = 0): List[List[TypeDef] | List[ValDef]] =

      def recur(numLeadParams: Int, firstClause: Boolean, prevIsTypeClause: Boolean): List[List[TypeDef] | List[ValDef]] =
        newLineOptWhenFollowedBy(LPAREN)
        newLineOptWhenFollowedBy(LBRACKET)
        if in.token == LPAREN then
          val paramsStart = in.offset
          val params = termParamClause(paramOwner, numLeadParams, firstClause)
          val lastClause = params.nonEmpty && params.head.mods.flags.is(Implicit)
          params :: (
            if lastClause then Nil
            else recur(numLeadParams + params.length, firstClause = false, prevIsTypeClause = false))
        else if in.token == LBRACKET then
          if prevIsTypeClause then
            syntaxError(
              em"Type parameter lists must be separated by a term or using parameter list",
              in.offset
            )
          typeParamClause(paramOwner) :: recur(numLeadParams, firstClause, prevIsTypeClause = true)
        else Nil
      end recur

      recur(numLeadParams, firstClause = true, prevIsTypeClause = false)
    end typeOrTermParamClauses


    /** ClsTypeParamClause::=  ‘[’ ClsTypeParam {‘,’ ClsTypeParam} ‘]’
     *  ClsTypeParam      ::=  {Annotation} [‘+’ | ‘-’]
     *                         id [HkTypeParamClause] TypeParamBounds
     *
     *  DefTypeParamClause::=  ‘[’ DefTypeParam {‘,’ DefTypeParam} ‘]’
     *  DefTypeParam      ::=  {Annotation}
     *                         [`sealed`]                                    -- under captureChecking
     *                         id [HkTypeParamClause] TypeParamBounds
     *
     *  TypTypeParamClause::=  ‘[’ TypTypeParam {‘,’ TypTypeParam} ‘]’
     *  TypTypeParam      ::=  {Annotation} id [HkTypePamClause] TypeBounds
     *
     *  HkTypeParamClause ::=  ‘[’ HkTypeParam {‘,’ HkTypeParam} ‘]’
     *  HkTypeParam       ::=  {Annotation} [‘+’ | ‘-’] (id [HkTypePamClause] | ‘_’) TypeBounds
     */
    def typeParamClause(paramOwner: ParamOwner): List[TypeDef] = inBracketsWithCommas {

      def checkVarianceOK(): Boolean =
        val ok = paramOwner.acceptsVariance
        if !ok then syntaxError(em"no `+/-` variance annotation allowed here")
        in.nextToken()
        ok

      def typeParam(): TypeDef = {
        val isAbstractOwner = paramOwner == ParamOwner.Type || paramOwner == ParamOwner.TypeParam
        val start = in.offset
        var mods = annotsAsMods() | Param
        if paramOwner == ParamOwner.Class || paramOwner == ParamOwner.CaseClass then
          mods |= PrivateLocal
        if Feature.ccEnabled && in.token == SEALED then
          if paramOwner == ParamOwner.Def then mods |= Sealed
          else syntaxError(em"`sealed` modifier only allowed for method type parameters")
          in.nextToken()
        if isIdent(nme.raw.PLUS) && checkVarianceOK() then
          mods |= Covariant
        else if isIdent(nme.raw.MINUS) && checkVarianceOK() then
          mods |= Contravariant
        atSpan(start, nameStart) {
          val name =
            if (isAbstractOwner && in.token == USCORE) {
              in.nextToken()
              WildcardParamName.fresh().toTypeName
            }
            else ident().toTypeName
          val hkparams = typeParamClauseOpt(ParamOwner.Type)
          val bounds = if (isAbstractOwner) typeBounds() else typeParamBounds(name)
          TypeDef(name, lambdaAbstract(hkparams, bounds)).withMods(mods)
        }
      }
      commaSeparated(() => typeParam())
    }

    def typeParamClauseOpt(paramOwner: ParamOwner): List[TypeDef] =
      if (in.token == LBRACKET) typeParamClause(paramOwner) else Nil

    /** ContextTypes   ::=  FunArgType {‘,’ FunArgType}
     */
    def contextTypes(paramOwner: ParamOwner, numLeadParams: Int, impliedMods: Modifiers): List[ValDef] =
      val tps = commaSeparated(() => paramTypeOf(toplevelTyp))
      var counter = numLeadParams
      def nextIdx = { counter += 1; counter }
      val paramFlags = if paramOwner.isClass then LocalParamAccessor else Param
      tps.map(makeSyntheticParameter(nextIdx, _, paramFlags | Synthetic | impliedMods.flags))

    /** ClsTermParamClause    ::=  ‘(’ ClsParams ‘)’ | UsingClsTermParamClause
     *  UsingClsTermParamClause::= ‘(’ ‘using’ [‘erased’] (ClsParams | ContextTypes) ‘)’
     *  ClsParams         ::=  ClsParam {‘,’ ClsParam}
     *  ClsParam          ::=  {Annotation} [{Modifier} (‘val’ | ‘var’)] Param
     *
     *  TypelessClause    ::= DefTermParamClause
     *                      | UsingParamClause
     *
     *  DefTermParamClause::= [nl] ‘(’ [DefTermParams] ‘)’
     *  UsingParamClause  ::=  ‘(’ ‘using’ (DefTermParams | ContextTypes) ‘)’
     *  DefImplicitClause ::=  [nl] ‘(’ ‘implicit’ DefTermParams ‘)’
     *  DefTermParams     ::=  DefTermParam {‘,’ DefTermParam}
     *  DefTermParam      ::=  {Annotation} [‘erased’] [‘inline’] Param
     *
     *  Param             ::=  id `:' ParamType [`=' Expr]
     *
     *  @return   the list of parameter definitions
     */
    def termParamClause(
      paramOwner: ParamOwner,
      numLeadParams: Int,                      // number of parameters preceding this clause
      firstClause: Boolean = false             // clause is the first in regular list of clauses
    ): List[ValDef] = {
      var impliedMods: Modifiers = EmptyModifiers

      def addParamMod(mod: () => Mod) = impliedMods = addMod(impliedMods, atSpan(in.skipToken()) { mod() })

      def paramMods() =
        if in.token == IMPLICIT then
          addParamMod(() => Mod.Implicit())
        else
          if isIdent(nme.using) then
            addParamMod(() => Mod.Given())

      def param(): ValDef = {
        val start = in.offset
        var mods = impliedMods.withAnnotations(annotations())
        if isErasedKw then
          mods = addModifier(mods)
        if paramOwner.isClass then
          mods = addFlag(modifiers(start = mods), ParamAccessor)
          mods =
            if in.token == VAL then
              in.nextToken()
              mods
            else if in.token == VAR then
              val mod = atSpan(in.skipToken()) { Mod.Var() }
              addMod(mods, mod)
            else
              if (!(mods.flags &~ (ParamAccessor | Inline | Erased | impliedMods.flags)).isEmpty)
                syntaxError(em"`val` or `var` expected")
              if firstClause && paramOwner == ParamOwner.CaseClass then mods
              else mods | PrivateLocal
        else {
          if (isIdent(nme.inline) && in.isSoftModifierInParamModifierPosition)
            mods = addModifier(mods)
          mods |= Param
        }
        atSpan(start, nameStart) {
          val name = ident()
          acceptColon()
          if (in.token == ARROW && paramOwner.isClass && !mods.is(Local))
            syntaxError(VarValParametersMayNotBeCallByName(name, mods.is(Mutable)))
              // needed?, it's checked later anyway
          val tpt = paramType()
          val default =
            if (in.token == EQUALS) { in.nextToken(); subExpr() }
            else EmptyTree
          if (impliedMods.mods.nonEmpty)
            impliedMods = impliedMods.withMods(Nil) // keep only flags, so that parameter positions don't overlap
          ValDef(name, tpt, default).withMods(mods)
        }
      }

      def checkVarArgsRules(vparams: List[ValDef]): Unit = vparams match {
        case Nil =>
        case _ :: Nil if paramOwner != ParamOwner.ExtensionPrefix =>
        case vparam :: rest =>
          vparam.tpt match {
            case PostfixOp(_, op) if op.name == tpnme.raw.STAR =>
              syntaxError(VarArgsParamMustComeLast(), vparam.tpt.span)
            case _ =>
          }
          checkVarArgsRules(rest)
      }

      // begin termParamClause
      inParensWithCommas {
        if in.token == RPAREN && paramOwner != ParamOwner.ExtensionPrefix && !impliedMods.is(Given)
        then Nil
        else
          val clause =
            if paramOwner == ParamOwner.ExtensionPrefix
                && !isIdent(nme.using) && !isIdent(nme.erased)
            then
              param() :: Nil
            else
              paramMods()
              if paramOwner.takesOnlyUsingClauses && !impliedMods.is(Given) then
                syntaxError(em"`using` expected")
              val (firstParamMod, isParams) =
                var mods = EmptyModifiers
                if in.lookahead.isColon then
                  (mods, true)
                else
                  if isErased then mods = addModifier(mods)
                  val isParams =
                    !impliedMods.is(Given)
                    || startParamTokens.contains(in.token)
                    || isIdent && (in.name == nme.inline || in.lookahead.isColon)
                  (mods, isParams)
              (if isParams then commaSeparated(() => param())
              else contextTypes(paramOwner, numLeadParams, impliedMods)) match {
                case Nil => Nil
                case (h :: t) => h.withAddedFlags(firstParamMod.flags) :: t
              }
          checkVarArgsRules(clause)
          clause
      }
    }

    /** ClsTermParamClauses   ::=  {ClsTermParamClause} [[nl] ‘(’ [‘implicit’] ClsParams ‘)’]
     *  TypelessClauses       ::=  TypelessClause {TypelessClause}
     *
     *  @return  The parameter definitions
     */
    def termParamClauses(paramOwner: ParamOwner, numLeadParams: Int = 0): List[List[ValDef]] =

      def recur(numLeadParams: Int, firstClause: Boolean): List[List[ValDef]] =
        newLineOptWhenFollowedBy(LPAREN)
        if in.token == LPAREN then
          val paramsStart = in.offset
          val params = termParamClause(paramOwner, numLeadParams, firstClause)
          val lastClause = params.nonEmpty && params.head.mods.flags.is(Implicit)
          params :: (
            if lastClause then Nil
            else recur(numLeadParams + params.length, firstClause = false))
        else Nil
      end recur

      recur(numLeadParams, firstClause = true)
    end termParamClauses

/* -------- DEFS ------------------------------------------- */

    def finalizeDef(md: MemberDef, mods: Modifiers, start: Int): md.ThisTree[Untyped] =
      md.withMods(mods).setComment(in.getDocComment(start))

    type ImportConstr = (Tree, List[ImportSelector]) => Tree

    /** Import  ::= `import' ImportExpr {‘,’ ImportExpr}
     *  Export  ::= `export' ImportExpr {‘,’ ImportExpr}
     */
    def importOrExportClause(leading: Token, mkTree: ImportConstr): List[Tree] = {
      val offset = accept(leading)
      commaSeparated(importExpr(leading, mkTree)) match {
        case t :: rest =>
          // The first import should start at the start offset of the keyword.
          val firstPos =
            if (t.span.exists) t.span.withStart(offset)
            else Span(offset, in.lastOffset)
          t.withSpan(firstPos) :: rest
        case nil => nil
      }
    }

    def exportClause() =
      importOrExportClause(EXPORT, Export(_,_))

    def importClause(outermost: Boolean = false) =
      importOrExportClause(IMPORT, mkImport(outermost))

    /** Create an import node and handle source version imports */
    def mkImport(outermost: Boolean = false): ImportConstr = (tree, selectors) =>
      val imp = Import(tree, selectors)
      languageImport(tree) match
        case Some(prefix) =>
          in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
          for case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors do
            if Feature.handleGlobalLanguageImport(prefix, imported) && !outermost then
              syntaxError(em"this language import is only allowed at the toplevel", id.span)
            if allSourceVersionNames.contains(imported) && prefix.isEmpty then
              if !outermost then
                syntaxError(em"source version import is only allowed at the toplevel", id.span)
              else if ctx.compilationUnit.sourceVersion.isDefined then
                syntaxError(em"duplicate source version import", id.span)
              else if illegalSourceVersionNames.contains(imported) then
                val candidate =
                  val nonMigration = imported.toString.replace("-migration", "")
                  validSourceVersionNames.find(_.show == nonMigration)
                val baseMsg = em"`$imported` is not a valid source version"
                val msg = candidate match
                  case Some(member) => baseMsg.append(i", did you mean language.`$member`?")
                  case _ => baseMsg
                syntaxError(msg, id.span)
              else
                ctx.compilationUnit.sourceVersion = Some(SourceVersion.valueOf(imported.toString))
        case None =>
      imp

    /** ImportExpr       ::=  SimpleRef {‘.’ id} ‘.’ ImportSpec
     *                     |  SimpleRef ‘as’ id
     *  ImportSpec       ::=  NamedSelector
     *                     |  WildcardSelector
     *                     | ‘{’ ImportSelectors ‘}’
     *  ImportSelectors  ::=  NamedSelector [‘,’ ImportSelectors]
     *                     |  WildCardSelector {‘,’ WildCardSelector}
     *  NamedSelector    ::=  id [‘as’ (id | ‘_’)]
     *  WildCardSelector ::=  ‘*' | ‘given’ [InfixType]
     */
    def importExpr(leading: Token, mkTree: ImportConstr): () => Tree =

      def exprName =
        (leading: @unchecked) match
          case EXPORT => "export"
          case IMPORT => "import"

      /** ‘*' | ‘_' */
      def wildcardSelector() =
        if in.token == USCORE && sourceVersion.isAtLeast(future) then
          report.errorOrMigrationWarning(
            em"`_` is no longer supported for a wildcard $exprName; use `*` instead${rewriteNotice(`future-migration`)}",
            in.sourcePos(),
            from = future)
          patch(source, Span(in.offset, in.offset + 1), "*")
        ImportSelector(atSpan(in.skipToken()) { Ident(nme.WILDCARD) })

      /** 'given [InfixType]' */
      def givenSelector() =
        ImportSelector(
          atSpan(in.skipToken()) { Ident(nme.EMPTY) },
          bound =
            if canStartInfixTypeTokens.contains(in.token) then rejectWildcardType(infixType())
            else EmptyTree)

      /** id [‘as’ (id | ‘_’) */
      def namedSelector(from: Ident) =
        if in.token == ARROW || isIdent(nme.as) then
          if in.token == ARROW && sourceVersion.isAtLeast(future) then
            report.errorOrMigrationWarning(
              em"The $exprName renaming `a => b` is no longer supported ; use `a as b` instead${rewriteNotice(`future-migration`)}",
              in.sourcePos(),
              from = future)
            patch(source, Span(in.offset, in.offset + 2),
                if testChar(in.offset - 1, ' ') && testChar(in.offset + 2, ' ') then "as"
                else " as ")
          atSpan(startOffset(from), in.skipToken()) {
            val to = if in.token == USCORE then wildcardIdent() else termIdent()
            ImportSelector(from, if to.name == nme.ERROR then EmptyTree else to)
          }
        else ImportSelector(from)

      def importSelector(idOK: Boolean): ImportSelector =
        atSpan(in.offset) {
          in.token match
            case USCORE => wildcardSelector()
            case GIVEN => givenSelector()
            case _ =>
              if isIdent(nme.raw.STAR) then wildcardSelector()
              else
                if !idOK then syntaxError(em"named ${exprName}s cannot follow wildcard ${exprName}s")
                namedSelector(termIdent())
        }

      def importSelectors(): List[ImportSelector] =
        var idOK = true
        commaSeparated { () =>
          val isWildcard = in.token == USCORE || in.token == GIVEN || isIdent(nme.raw.STAR)
          try importSelector(idOK)
          finally idOK &= !isWildcard
        }

      def importSelection(qual: Tree): Tree =
        if in.isIdent(nme.as) && qual.isInstanceOf[RefTree] then
          qual match
            case Select(qual1, name) =>
              val from = Ident(name).withSpan(Span(qual.span.point, qual.span.end, 0))
              mkTree(qual1, namedSelector(from) :: Nil)
            case qual: Ident =>
              mkTree(EmptyTree, namedSelector(qual) :: Nil)
        else
          accept(DOT)
          in.token match
            case USCORE =>
              mkTree(qual, wildcardSelector() :: Nil)
            case GIVEN =>
              mkTree(qual, givenSelector() :: Nil)
            case LBRACE =>
              mkTree(qual, inBraces(importSelectors()))
            case _ =>
              if isIdent(nme.raw.STAR) then
                mkTree(qual, wildcardSelector() :: Nil)
              else
                val start = in.offset
                val name = ident()
                if in.token == DOT then
                  importSelection(atSpan(startOffset(qual), start) { Select(qual, name) })
                else
                  mkTree(qual, namedSelector(atSpan(start) { Ident(name) }) :: Nil)
      end importSelection

      () => atSpan(in.offset) { importSelection(simpleRef()) }
    end importExpr

    /** Def      ::= val PatDef
     *             | var VarDef
     *             | def DefDef
     *             | type {nl} TypeDef
     *             | TmplDef
     *  EnumCase ::= `case' (id ClassConstr [`extends' ConstrApps]] | ids)
     */
    def defOrDcl(start: Int, mods: Modifiers): Tree = in.token match {
      case VAL =>
        in.nextToken()
        patDefOrDcl(start, mods)
      case VAR =>
        val mod = atSpan(in.skipToken()) { Mod.Var() }
        val mod1 = addMod(mods, mod)
        patDefOrDcl(start, mod1)
      case DEF =>
        defDefOrDcl(start, in.skipToken(mods))
      case TYPE =>
        typeDefOrDcl(start, in.skipToken(mods))
      case CASE if inEnum =>
        enumCase(start, mods)
      case _ =>
        tmplDef(start, mods)
    }

    /** PatDef  ::=  ids [‘:’ Type] [‘=’ Expr]
     *            |  Pattern2 [‘:’ Type] [‘=’ Expr]
     *  VarDef  ::=  PatDef
     *            |  id {`,' id} `:' Type `=' `_' (deprecated in 3.x)
     */
    def patDefOrDcl(start: Offset, mods: Modifiers): Tree = atSpan(start, nameStart) {
      if in.token != USCORE && isKeyword(in.token) then
        syntaxError(ExpectedTokenButFound(IDENTIFIER, in.token), Span(in.offset))
      val first = pattern2()
      var lhs = first match {
        case id: Ident if in.token == COMMA =>
          in.nextToken()
          id :: commaSeparated(() => termIdent())
        case _ =>
          first :: Nil
      }
      val tpt = typedOpt()
      val rhs =
        if tpt.isEmpty || in.token == EQUALS then
          accept(EQUALS)
          val rhsOffset = in.offset
          subExpr() match
            case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name
                && !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident]) =>
              if sourceVersion.isAtLeast(future) then
                deprecationWarning(
                  em"""`= _` has been deprecated; use `= uninitialized` instead.
                      |`uninitialized` can be imported with `scala.compiletime.uninitialized`.""",
                  rhsOffset)
              placeholderParams = placeholderParams.tail
              atSpan(rhs0.span) { Ident(nme.WILDCARD) }
            case rhs0 => rhs0
        else EmptyTree
      lhs match {
        case IdPattern(id, t) :: Nil if t.isEmpty =>
          val vdef = ValDef(id.name.asTermName, tpt, rhs)
          if (isBackquoted(id)) vdef.pushAttachment(Backquoted, ())
          finalizeDef(vdef, mods, start)
        case _ =>
          def isAllIds = lhs.forall {
            case IdPattern(id, t) => t.isEmpty
            case _ => false
          }
          val rhs2 =
            if rhs.isEmpty && !isAllIds then
              val start = in.lastOffset
              syntaxError(ExpectedTokenButFound(EQUALS, in.token), Span(in.lastOffset))
              errorTermTree(start)
            else
              rhs
          PatDef(mods, lhs, tpt, rhs2)
      }
    }

    /** DefDef  ::=  DefSig [‘:’ Type] [‘=’ Expr]
     *            |  this TypelessClauses [DefImplicitClause] `=' ConstrExpr
     *  DefSig  ::=  id [DefTypeParamClause] DefTermParamClauses
     *
     * if clauseInterleaving is enabled:
     *  DefSig  ::=  id [DefParamClauses] [DefImplicitClause]
     */
    def defDefOrDcl(start: Offset, mods: Modifiers, numLeadParams: Int = 0): DefDef = atSpan(start, nameStart) {

      def scala2ProcedureSyntax(resultTypeStr: String) =
        def toInsert =
          if in.token == LBRACE then s"$resultTypeStr ="
          else ": Unit "  // trailing space ensures that `def f()def g()` works.
        if migrateTo3 then
          report.errorOrMigrationWarning(
            em"Procedure syntax no longer supported; `$toInsert` should be inserted here",
            in.sourcePos(), from = `3.0`)
          patch(source, Span(in.lastOffset), toInsert)
          true
        else
          false

      if (in.token == THIS) {
        in.nextToken()
        val vparamss = termParamClauses(ParamOwner.Def, numLeadParams)
        if (vparamss.isEmpty || vparamss.head.take(1).exists(_.mods.isOneOf(GivenOrImplicit)))
          in.token match {
            case LBRACKET   => syntaxError(em"no type parameters allowed here")
            case EOF        => incompleteInputError(AuxConstructorNeedsNonImplicitParameter())
            case _          => syntaxError(AuxConstructorNeedsNonImplicitParameter(), nameStart)
          }
        if (migrateTo3) newLineOptWhenFollowedBy(LBRACE)
        val rhs = {
          if (!(in.token == LBRACE && scala2ProcedureSyntax(""))) accept(EQUALS)
          atSpan(in.offset) { subPart(constrExpr) }
        }
        makeConstructor(Nil, vparamss, rhs).withMods(mods).setComment(in.getDocComment(start))
      }
      else {
        val mods1 = addFlag(mods, Method)
        val ident = termIdent()
        var name = ident.name.asTermName
        val paramss =
          if in.featureEnabled(Feature.clauseInterleaving) then
            // If you are making interleaving stable manually, please refer to the PR introducing it instead, section "How to make non-experimental"
            typeOrTermParamClauses(ParamOwner.Def, numLeadParams)
          else
            val tparams = typeParamClauseOpt(ParamOwner.Def)
            val vparamss = termParamClauses(ParamOwner.Def, numLeadParams)

            joinParams(tparams, vparamss)

        var tpt = fromWithinReturnType { typedOpt() }

        if (migrateTo3) newLineOptWhenFollowedBy(LBRACE)
        val rhs =
          if in.token == EQUALS then
            in.nextToken()
            subExpr()
          else if !tpt.isEmpty then
            EmptyTree
          else if scala2ProcedureSyntax(": Unit") then
            tpt = scalaUnit
            if (in.token == LBRACE) expr()
            else EmptyTree
          else
            if (!isExprIntro) syntaxError(MissingReturnType(), in.lastOffset)
            accept(EQUALS)
            expr()

        val ddef = DefDef(name, paramss, tpt, rhs)
        if (isBackquoted(ident)) ddef.pushAttachment(Backquoted, ())
        finalizeDef(ddef, mods1, start)
      }
    }

    /** ConstrExpr      ::=  SelfInvocation
     *                    |  `{' SelfInvocation {semi BlockStat} `}'
     */
    val constrExpr: () => Tree = () =>
      if in.isNestedStart then
        atSpan(in.offset) {
          inBracesOrIndented {
            val stats = selfInvocation() :: (
              if (isStatSep) { in.nextToken(); blockStatSeq() }
              else Nil)
            Block(stats, unitLiteral)
          }
        }
      else Block(selfInvocation() :: Nil, unitLiteral)

    /** SelfInvocation  ::= this ArgumentExprs {ArgumentExprs}
     */
    def selfInvocation(): Tree =
      atSpan(accept(THIS)) {
        argumentStart()
        argumentExprss(mkApply(Ident(nme.CONSTRUCTOR), argumentExprs()))
      }

    /** TypeDef ::=  id [TypeParamClause] {FunParamClause} TypeBounds [‘=’ Type]
     */
    def typeDefOrDcl(start: Offset, mods: Modifiers): Tree = {
      newLinesOpt()
      atSpan(start, nameStart) {
        val nameIdent = typeIdent()
        val tparams = typeParamClauseOpt(ParamOwner.Type)
        val vparamss = funParamClauses()
        def makeTypeDef(rhs: Tree): Tree = {
          val rhs1 = lambdaAbstractAll(tparams :: vparamss, rhs)
          val tdef = TypeDef(nameIdent.name.toTypeName, rhs1)
          if (nameIdent.isBackquoted)
            tdef.pushAttachment(Backquoted, ())
          finalizeDef(tdef, mods, start)
        }
        in.token match {
          case EQUALS =>
            in.nextToken()
            makeTypeDef(toplevelTyp())
          case SUBTYPE | SUPERTYPE =>
            val bounds = typeBounds()
            if (in.token == EQUALS) {
              val eqOffset = in.skipToken()
              var rhs = toplevelTyp()
              rhs match {
                case mtt: MatchTypeTree =>
                  bounds match {
                    case TypeBoundsTree(EmptyTree, upper, _) =>
                      rhs = MatchTypeTree(upper, mtt.selector, mtt.cases)
                    case _ =>
                      syntaxError(em"cannot combine lower bound and match type alias", eqOffset)
                  }
                case _ =>
                  if mods.is(Opaque) then
                    rhs = TypeBoundsTree(bounds.lo, bounds.hi, rhs)
                  else
                    syntaxError(em"cannot combine bound and alias", eqOffset)
              }
              makeTypeDef(rhs)
            }
            else makeTypeDef(bounds)
          case SEMI | NEWLINE | NEWLINES | COMMA | RBRACE | OUTDENT | EOF =>
            makeTypeDef(typeBounds())
          case _ =>
            syntaxErrorOrIncomplete(ExpectedTypeBoundOrEquals(in.token))
            return EmptyTree // return to avoid setting the span to EmptyTree
        }
      }
    }

    /** TmplDef ::=  ([‘case’] ‘class’ | ‘trait’) ClassDef
     *            |  [‘case’] ‘object’ ObjectDef
     *            |  ‘enum’ EnumDef
     *            |  ‘given’ GivenDef
     */
    def tmplDef(start: Int, mods: Modifiers): Tree =
      in.token match {
        case TRAIT =>
          classDef(start, in.skipToken(addFlag(mods, Trait)))
        case CLASS =>
          classDef(start, in.skipToken(mods))
        case CASECLASS =>
          classDef(start, in.skipToken(mods | Case))
        case OBJECT =>
          objectDef(start, in.skipToken(mods | Module))
        case CASEOBJECT =>
          objectDef(start, in.skipToken(mods | Case | Module))
        case ENUM =>
          enumDef(start, in.skipToken(mods | Enum))
        case GIVEN =>
          givenDef(start, mods, atSpan(in.skipToken()) { Mod.Given() })
        case _ =>
          val start = in.lastOffset
          syntaxErrorOrIncomplete(ExpectedStartOfTopLevelDefinition())
          mods.annotations match {
            case head :: Nil => head
            case Nil => EmptyTree
            case all => Block(all, errorTermTree(start))
          }
      }

    /** ClassDef ::= id ClassConstr TemplateOpt
     */
    def classDef(start: Offset, mods: Modifiers): TypeDef = atSpan(start, nameStart) {
      classDefRest(start, mods, ident().toTypeName)
    }

    def classDefRest(start: Offset, mods: Modifiers, name: TypeName): TypeDef =
      val constr = classConstr(if mods.is(Case) then ParamOwner.CaseClass else ParamOwner.Class)
      val templ = templateOpt(constr)
      finalizeDef(TypeDef(name, templ), mods, start)

    /** ClassConstr ::= [ClsTypeParamClause] [ConstrMods] ClsTermParamClauses
     */
    def classConstr(paramOwner: ParamOwner): DefDef = atSpan(in.lastOffset) {
      val tparams = typeParamClauseOpt(paramOwner)
      val cmods = fromWithinClassConstr(constrModsOpt())
      val vparamss = termParamClauses(paramOwner)
      makeConstructor(tparams, vparamss).withMods(cmods)
    }

    /** ConstrMods        ::=  {Annotation} [AccessModifier]
     */
    def constrModsOpt(): Modifiers =
      modifiers(accessModifierTokens, annotsAsMods())

    /** ObjectDef       ::= id TemplateOpt
     */
    def objectDef(start: Offset, mods: Modifiers): ModuleDef = atSpan(start, nameStart) {
      val name = ident()
      val templ = templateOpt(emptyConstructor)
      finalizeDef(ModuleDef(name, templ), mods, start)
    }

    private def checkAccessOnly(mods: Modifiers, where: String): Modifiers =
      val mods1 = mods & (AccessFlags | Enum)
      if mods1 ne mods then
        syntaxError(em"Only access modifiers are allowed on enum $where")
      mods1

    /**  EnumDef ::=  id ClassConstr InheritClauses EnumBody
     */
    def enumDef(start: Offset, mods: Modifiers): TypeDef = atSpan(start, nameStart) {
      val mods1 = checkAccessOnly(mods, "definitions")
      val modulName = ident()
      val clsName = modulName.toTypeName
      val constr = classConstr(ParamOwner.Class)
      val templ = template(constr, isEnum = true)
      finalizeDef(TypeDef(clsName, templ), mods1, start)
    }

    /** EnumCase = `case' (id ClassConstr [`extends' ConstrApps] | ids)
     */
    def enumCase(start: Offset, mods: Modifiers): DefTree = {
      val mods1 = checkAccessOnly(mods, "cases") | EnumCase
      accept(CASE)

      atSpan(start, nameStart) {
        val id = termIdent()
        if (in.token == COMMA) {
          in.nextToken()
          val ids = commaSeparated(() => termIdent())
          if ctx.settings.WenumCommentDiscard.value then
            in.getDocComment(start).foreach: comm =>
              warning(
                em"""Ambiguous Scaladoc comment on multiple cases is ignored.
                     |Remove the comment or make separate cases to add Scaladoc comments to each of them.""",
                comm.span.start
              )

          PatDef(mods1, id :: ids, TypeTree(), EmptyTree)
        }
        else {
          val caseDef =
            if (in.token == LBRACKET || in.token == LPAREN || in.token == AT || isModifier) {
              val clsName = id.name.toTypeName
              val constr = classConstr(ParamOwner.CaseClass)
              TypeDef(clsName, caseTemplate(constr))
            }
            else
              ModuleDef(id.name.toTermName, caseTemplate(emptyConstructor))
          finalizeDef(caseDef, mods1, start)
        }
      }
    }

    /** [`extends' ConstrApps] */
    def caseTemplate(constr: DefDef): Template = {
      val parents =
        if (in.token == EXTENDS) {
          in.nextToken()
          constrApps()
        }
        else Nil
      Template(constr, parents, Nil, EmptyValDef, Nil)
    }

    def checkExtensionMethod(tparams: List[Tree],
        vparamss: List[List[Tree]], stat: Tree): Unit = stat match {
      case stat: DefDef =>
        if stat.mods.is(ExtensionMethod) && vparamss.nonEmpty then
          syntaxError(em"no extension method allowed here since leading parameter was already given", stat.span)
        else if !stat.mods.is(ExtensionMethod) && vparamss.isEmpty then
          syntaxError(em"an extension method is required here", stat.span)
        else if tparams.nonEmpty && stat.leadingTypeParams.nonEmpty then
          syntaxError(em"extension method cannot have type parameters since some were already given previously",
            stat.leadingTypeParams.head.span)
        else if stat.rhs.isEmpty then
          syntaxError(em"extension method cannot be abstract", stat.span)
      case EmptyTree =>
      case stat =>
        syntaxError(em"extension clause can only define methods", stat.span)
    }

    /** GivenDef          ::=  [GivenSig] (AnnotType [‘=’ Expr] | StructuralInstance)
     *  GivenSig          ::=  [id] [DefTypeParamClause] {UsingParamClauses} ‘:’
     */
    def givenDef(start: Offset, mods: Modifiers, givenMod: Mod) = atSpan(start, nameStart) {
      var mods1 = addMod(mods, givenMod)
      val nameStart = in.offset
      val name = if isIdent && followingIsGivenSig() then ident() else EmptyTermName

      val gdef =
        val tparams = typeParamClauseOpt(ParamOwner.Given)
        newLineOpt()
        val vparamss =
          if in.token == LPAREN && (in.lookahead.isIdent(nme.using) || name != EmptyTermName)
          then termParamClauses(ParamOwner.Given)
          else Nil
        newLinesOpt()
        val noParams = tparams.isEmpty && vparamss.isEmpty
        if !(name.isEmpty && noParams) then acceptColon()
        val parents =
          if isSimpleLiteral then rejectWildcardType(annotType()) :: Nil
          else refinedTypeRest(constrApp()) :: withConstrApps()
        val parentsIsType = parents.length == 1 && parents.head.isType
        if in.token == EQUALS && parentsIsType then
          accept(EQUALS)
          mods1 |= Final
          if noParams && !mods.is(Inline) then
            mods1 |= Lazy
            ValDef(name, parents.head, subExpr())
          else
            DefDef(name, joinParams(tparams, vparamss), parents.head, subExpr())
        else if (isStatSep || isStatSeqEnd) && parentsIsType then
          if name.isEmpty then
            syntaxError(em"anonymous given cannot be abstract")
          DefDef(name, joinParams(tparams, vparamss), parents.head, EmptyTree)
        else
          val tparams1 = tparams.map(tparam => tparam.withMods(tparam.mods | PrivateLocal))
          val vparamss1 = vparamss.map(_.map(vparam =>
            vparam.withMods(vparam.mods &~ Param | ParamAccessor | Protected)))
          val constr = makeConstructor(tparams1, vparamss1)
          val templ =
            if isStatSep || isStatSeqEnd then Template(constr, parents, Nil, EmptyValDef, Nil)
            else withTemplate(constr, parents)
          if noParams && !mods.is(Inline) then ModuleDef(name, templ)
          else TypeDef(name.toTypeName, templ)
      end gdef
      finalizeDef(gdef, mods1, start)
    }

    /** Extension  ::=  ‘extension’ [DefTypeParamClause] {UsingParamClause} ‘(’ DefTermParam ‘)’
     *                  {UsingParamClause} ExtMethods
     */
    def extension(): ExtMethods =
      val start = in.skipToken()
      val tparams = typeParamClauseOpt(ParamOwner.ExtensionPrefix)
      val leadParamss = ListBuffer[List[ValDef]]()
      def numLeadParams = leadParamss.map(_.length).sum
      while
        val extParams = termParamClause(ParamOwner.ExtensionPrefix, numLeadParams)
        leadParamss += extParams
        isUsingClause(extParams)
      do ()
      leadParamss ++= termParamClauses(ParamOwner.ExtensionFollow, numLeadParams)
      if in.isColon then
        syntaxError(em"no `:` expected here")
        in.nextToken()
      val methods: List[Tree] =
        if in.token == EXPORT then
          exportClause()
        else if isDefIntro(modifierTokens) then
          extMethod(numLeadParams) :: Nil
        else
          in.observeIndented()
          newLineOptWhenFollowedBy(LBRACE)
          if in.isNestedStart then inDefScopeBraces(extMethods(numLeadParams))
          else { syntaxErrorOrIncomplete(em"Extension without extension methods") ; Nil }
      val result = atSpan(start)(ExtMethods(joinParams(tparams, leadParamss.toList), methods))
      val comment = in.getDocComment(start)
      if comment.isDefined then
        for case meth: DefDef <- methods do
          if !meth.rawComment.isDefined then meth.setComment(comment)
      result
    end extension

    /**  ExtMethod  ::=  {Annotation [nl]} {Modifier} ‘def’ DefDef
     *                |  Export
     */
    def extMethod(numLeadParams: Int): DefDef =
      val start = in.offset
      val mods = defAnnotsMods(modifierTokens)
      accept(DEF)
      defDefOrDcl(start, mods, numLeadParams)

    /** ExtMethods ::=  ExtMethod | [nl] ‘{’ ExtMethod {semi ExtMethod ‘}’
     */
    def extMethods(numLeadParams: Int): List[Tree] = checkNoEscapingPlaceholders {
      val meths = new ListBuffer[Tree]
      while
        val start = in.offset
        if in.token == EXPORT then
          meths ++= exportClause()
        else
          val mods = defAnnotsMods(modifierTokens)
          if in.token != EOF then
            accept(DEF)
            meths += defDefOrDcl(start, mods, numLeadParams)
        in.token != EOF && statSepOrEnd(meths, what = "extension method")
      do ()
      if meths.isEmpty then syntaxErrorOrIncomplete(em"`def` expected")
      meths.toList
    }

/* -------- TEMPLATES ------------------------------------------- */

    /** ConstrApp  ::=  SimpleType1 {Annotation} {ParArgumentExprs}
     */
    val constrApp: () => Tree = () =>
      val t = rejectWildcardType(annotTypeRest(simpleType1()),
        fallbackTree = Ident(tpnme.ERROR))
        // Using Ident(tpnme.ERROR) to avoid causing cascade errors on non-user-written code
      if in.token == LPAREN then parArgumentExprss(wrapNew(t)) else t

    /** ConstrApps  ::=  ConstrApp ({‘,’ ConstrApp} | {‘with’ ConstrApp})
     */
    def constrApps(exclude: Token = EMPTY): List[Tree] =
      val t = constrApp()
      val ts =
        val tok = in.token
        if (tok == WITH || tok == COMMA) && tok != exclude then
          in.nextToken()
          constrApps(exclude = if tok == WITH then COMMA else WITH)
        else Nil
      t :: ts

    /** `{`with` ConstrApp} but no EOL allowed after `with`.
     */
    def withConstrApps(): List[Tree] =
      def isTemplateStart =
        val la = in.lookahead
        la.isAfterLineEnd || la.token == LBRACE
      if in.token == WITH && !isTemplateStart then
        in.nextToken()
        constrApp() :: withConstrApps()
      else Nil

    /** Template          ::=  InheritClauses [TemplateBody]
     *  InheritClauses    ::=  [‘extends’ ConstrApps] [‘derives’ QualId {‘,’ QualId}]
     */
    def template(constr: DefDef, isEnum: Boolean = false): Template = {
      val parents =
        if (in.token == EXTENDS) {
          in.nextToken()
          if (in.token == LBRACE || in.token == COLONeol) {
            report.errorOrMigrationWarning(
              em"`extends` must be followed by at least one parent",
              in.sourcePos(), from = `3.0`)
            Nil
          }
          else constrApps()
        }
        else Nil
      newLinesOptWhenFollowedBy(nme.derives)
      val derived =
        if (isIdent(nme.derives)) {
          in.nextToken()
          commaSeparated(() => convertToTypeId(qualId()))
        }
        else Nil
      possibleTemplateStart()
      if isEnum then
        val (self, stats) = withinEnum(templateBody(parents))
        Template(constr, parents, derived, self, stats)
      else
        templateBodyOpt(constr, parents, derived)
    }

    /** TemplateOpt = [Template]
     */
    def templateOpt(constr: DefDef): Template =
      newLinesOptWhenFollowedBy(nme.derives)
      if in.token == EXTENDS || isIdent(nme.derives) then
        template(constr)
      else
        possibleTemplateStart()
        if in.isNestedStart then
          template(constr)
        else
          checkNextNotIndented()
          Template(constr, Nil, Nil, EmptyValDef, Nil)

    /** TemplateBody ::=  [nl] `{' TemplateStatSeq `}'
     *  EnumBody     ::=  [nl] ‘{’ [SelfType] EnumStat {semi EnumStat} ‘}’
     */
    def templateBodyOpt(constr: DefDef, parents: List[Tree], derived: List[Tree]): Template =
      val (self, stats) =
        if in.isNestedStart then
          templateBody(parents)
        else
          checkNextNotIndented()
          (EmptyValDef, Nil)
      Template(constr, parents, derived, self, stats)

    def templateBody(parents: List[Tree], rewriteWithColon: Boolean = true): (ValDef, List[Tree]) =
      val r = inDefScopeBraces(templateStatSeq(), rewriteWithColon)
      if in.token == WITH && parents.isEmpty then
        syntaxError(EarlyDefinitionsNotSupported())
        in.nextToken()
        template(emptyConstructor)
      r

    /** with Template, with EOL <indent> interpreted */
    def withTemplate(constr: DefDef, parents: List[Tree]): Template =
      accept(WITH)
      val (self, stats) = templateBody(parents, rewriteWithColon = false)
      Template(constr, parents, Nil, self, stats)
        .withSpan(Span(constr.span.orElse(parents.head.span).start, in.lastOffset))

/* -------- STATSEQS ------------------------------------------- */

    /** Create a tree representing a packaging */
    def makePackaging(start: Int, pkg: Tree, stats: List[Tree]): PackageDef = pkg match {
      case x: RefTree => atSpan(start, pointOffset(pkg))(PackageDef(x, stats))
    }

    /** Packaging ::= package QualId [nl] `{' TopStatSeq `}'
     */
    def packaging(start: Int): Tree =
      val pkg = qualId()
      possibleTemplateStart()
      val stats = inDefScopeBraces(topStatSeq(), rewriteWithColon = true)
      makePackaging(start, pkg, stats)

    /** TopStatSeq ::= TopStat {semi TopStat}
     *  TopStat ::= Import
     *            | Export
     *            | Annotations Modifiers Def
     *            | Packaging
     *            | package object objectDef
     *            | Extension
     *            |
     */
    def topStatSeq(outermost: Boolean = false): List[Tree] = {
      val stats = new ListBuffer[Tree]
      while
        var empty = false
        if (in.token == PACKAGE) {
          val start = in.skipToken()
          if (in.token == OBJECT) {
            in.nextToken()
            stats += objectDef(start, Modifiers(Package))
          }
          else stats += packaging(start)
        }
        else if (in.token == IMPORT)
          stats ++= importClause(outermost)
        else if (in.token == EXPORT)
          stats ++= exportClause()
        else if isIdent(nme.extension) && followingIsExtension() then
          stats += extension()
        else if isDefIntro(modifierTokens) then
          stats +++= defOrDcl(in.offset, defAnnotsMods(modifierTokens))
        else
          empty = true
        statSepOrEnd(stats, noPrevStat = empty, "toplevel definition")
      do ()
      stats.toList
    }

    /** SelfType ::=  id [‘:’ InfixType] ‘=>’
     *            |  ‘this’ ‘:’ InfixType ‘=>’
     */
    def selfType(): ValDef =
      if (in.isIdent || in.token == THIS)
        && (in.lookahead.isColon && followingIsSelfType()
            || in.lookahead.token == ARROW)
      then
        atSpan(in.offset) {
          val selfName =
            if in.token == THIS then
              in.nextToken()
              nme.WILDCARD
            else ident()
          val selfTpt =
            if in.isColon then
              in.nextToken()
              infixType()
            else
              if selfName == nme.WILDCARD then accept(COLONfollow)
              TypeTree()
          if in.token == ARROW then
            in.token = SELFARROW // suppresses INDENT insertion after `=>`
            in.nextToken()
          else
            syntaxError(em"`=>` expected after self type")
          makeSelfDef(selfName, selfTpt)
        }
      else EmptyValDef

    /** TemplateStatSeq  ::= [SelfType] TemplateStat {semi TemplateStat}
     *  TemplateStat     ::= Import
     *                     | Export
     *                     | Annotations Modifiers Def
     *                     | Extension
     *                     | Expr1
     *                     |
     *  EnumStat         ::= TemplateStat
     *                     | Annotations Modifiers EnumCase
     */
    def templateStatSeq(): (ValDef, List[Tree]) = checkNoEscapingPlaceholders {
      val stats = new ListBuffer[Tree]
      val self = selfType()
      while
        var empty = false
        if (in.token == IMPORT)
          stats ++= importClause()
        else if (in.token == EXPORT)
          stats ++= exportClause()
        else if isIdent(nme.extension) && followingIsExtension() then
          stats += extension()
        else if (isDefIntro(modifierTokensOrCase))
          stats +++= defOrDcl(in.offset, defAnnotsMods(modifierTokens))
        else if (isExprIntro)
          stats += expr1()
        else
          empty = true
        statSepOrEnd(stats, noPrevStat = empty)
      do ()
      (self, if stats.isEmpty then List(EmptyTree) else stats.toList)
    }

    /** RefineStatSeq    ::=  RefineStat {semi RefineStat}
     *  RefineStat       ::=  ‘val’ VarDef
     *                     |  ‘def’ DefDef
     *                     |  ‘type’ {nl} TypeDef
     *  (in reality we admit class defs and vars and filter them out afterwards in `checkLegal`)
     */
    def refineStatSeq(): List[Tree] = {
      val stats = new ListBuffer[Tree]
      def checkLegal(tree: Tree): List[Tree] =
        def ok = tree :: Nil
        def fail(msg: Message) =
          syntaxError(msg, tree.span)
          Nil
        tree match
          case tree: ValDef if tree.mods.is(Mutable) =>
            fail(em"""refinement cannot be a mutable var.
                     |You can use an explicit getter ${tree.name} and setter ${tree.name}_= instead""")
          case tree: MemberDef if !(tree.mods.flags & ModifierFlags).isEmpty =>
            fail(em"refinement cannot be ${(tree.mods.flags & ModifierFlags).flagStrings().mkString("`", "`, `", "`")}")
          case tree: DefDef if tree.termParamss.nestedExists(!_.rhs.isEmpty) =>
            fail(em"refinement cannot have default arguments")
          case tree: ValOrDefDef =>
            if tree.rhs.isEmpty then ok
            else fail(em"refinement cannot have a right-hand side")
          case tree: TypeDef =>
            if !tree.isClassDef then ok
            else fail(em"refinement cannot be a class or trait")
          case _ =>
            fail(em"this kind of definition cannot be a refinement")

      while
        val dclFound = isDclIntro
        if dclFound then
          stats ++= checkLegal(defOrDcl(in.offset, Modifiers()))
        var what = "declaration"
        if inFunReturnType then what += " (possible cause: missing `=` in front of current method body)"
        statSepOrEnd(stats, noPrevStat = !dclFound, what)
      do ()
      stats.toList
    }

    def localDef(start: Int, implicitMods: Modifiers = EmptyModifiers): Tree = {
      var mods = defAnnotsMods(localModifierTokens)
      for (imod <- implicitMods.mods) mods = addMod(mods, imod)
      if (mods.is(Final))
        // A final modifier means the local definition is "class-like".  // FIXME: Deal with modifiers separately
        tmplDef(start, mods)
      else
        defOrDcl(start, mods)
    }

    /** BlockStatSeq ::= { BlockStat semi } [Expr]
     *  BlockStat    ::= Import
     *                 | Annotations [implicit] [lazy] Def
     *                 | Annotations LocalModifiers TmplDef
     *                 | Extension
     *                 | Expr1
     *                 |
     */
    def blockStatSeq(): List[Tree] = checkNoEscapingPlaceholders {
      val stats = new ListBuffer[Tree]
      while
        var empty = false
        if (in.token == IMPORT)
          stats ++= importClause()
        else if (isExprIntro)
          stats += expr(Location.InBlock)
        else if in.token == IMPLICIT && !in.inModifierPosition() then
          stats += closure(in.offset, Location.InBlock, modifiers(BitSet(IMPLICIT)))
        else if isIdent(nme.extension) && followingIsExtension() then
          stats += extension()
        else if isDefIntro(localModifierTokens, excludedSoftModifiers = Set(nme.`opaque`)) then
          stats +++= localDef(in.offset)
        else
          empty = true
        statSepOrEnd(stats, noPrevStat = empty, altEnd = CASE)
      do ()
      stats.toList
    }

    /** CompilationUnit ::= {package QualId semi} TopStatSeq
     */
    def compilationUnit(): Tree = checkNoEscapingPlaceholders {
      def topstats(): List[Tree] = {
        val ts = new ListBuffer[Tree]
        while (in.token == SEMI) in.nextToken()
        val start = in.offset
        if (in.token == PACKAGE) {
          in.nextToken()
          if (in.token == OBJECT) {
            in.nextToken()
            ts += objectDef(start, Modifiers(Package))
            if (in.token != EOF) {
              statSepOrEnd(ts, what = "toplevel definition")
              ts ++= topStatSeq()
            }
          }
          else
            val pkg = qualId()
            var continue = false
            possibleTemplateStart()
            if in.token == EOF then
              ts += makePackaging(start, pkg, List())
            else if in.isNestedStart then
              ts += inDefScopeBraces(makePackaging(start, pkg, topStatSeq()), rewriteWithColon = true)
              continue = true
            else
              acceptStatSep()
              ts += makePackaging(start, pkg, topstats())
            if continue then
              statSepOrEnd(ts, what = "toplevel definition")
              ts ++= topStatSeq()
        }
        else
          ts ++= topStatSeq(outermost = true)

        ts.toList
      }

      topstats() match {
        case List(stat @ PackageDef(_, _)) => stat
        case Nil => EmptyTree  // without this case we'd get package defs without positions
        case stats => PackageDef(Ident(nme.EMPTY_PACKAGE), stats)
      }
    }
  }

  /** OutlineParser parses top-level declarations in `source` to find declared classes, ignoring their bodies (which
   *  must only have balanced braces). This is used to map class names to defining sources.
   */
  class OutlineParser(source: SourceFile)(using Context) extends Parser(source) with OutlineParserCommon {

    def skipBracesHook(): Option[Tree] =
      if (in.token == XMLSTART) Some(xmlLiteral()) else None

    override def blockExpr(): Tree = {
      skipBraces()
      EmptyTree
    }

    override def templateBody(parents: List[Tree], rewriteWithColon: Boolean): (ValDef, List[Thicket]) = {
      skipBraces()
      (EmptyValDef, List(EmptyTree))
    }
  }
}