Parse unary operators as regular identifiers when backquoted

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2293,10 +2293,10 @@ object Parsers {
         isOperator = !(location.inArgs && followingIsVararg()))
 
     /** PrefixExpr       ::= [PrefixOperator'] SimpleExpr
-     *  PrefixOperator   ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
+     *  PrefixOperator   ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’ (if not backquoted)
      */
     val prefixExpr: Location => Tree = location =>
-      if isIdent && nme.raw.isUnary(in.name)
+      if in.token == IDENTIFIER && nme.raw.isUnary(in.name)
          && in.canStartExprTokens.contains(in.lookahead.token)
       then
         val start = in.offset

docs/_docs/internals/syntax.md

@@ -254,7 +254,7 @@ InfixExpr         ::=  PrefixExpr
                     |  InfixExpr MatchClause
 MatchClause       ::=  ‘match’ <<< CaseClauses >>>                              Match(expr, cases)
 PrefixExpr        ::=  [PrefixOperator] SimpleExpr                              PrefixOp(expr, op)
-PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
+PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’                                    -- unless backquoted
 SimpleExpr        ::=  SimpleRef
                     |  Literal
                     |  ‘_’

docs/_docs/reference/syntax.md

@@ -252,7 +252,7 @@ InfixExpr         ::=  PrefixExpr
                     |  InfixExpr MatchClause
 MatchClause       ::=  ‘match’ <<< CaseClauses >>>
 PrefixExpr        ::=  [PrefixOperator] SimpleExpr
-PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’
+PrefixOperator    ::=  ‘-’ | ‘+’ | ‘~’ | ‘!’                                    -- unless backquoted
 SimpleExpr        ::=  SimpleRef
                     |  Literal
                     |  ‘_’

tests/neg/unary-with-type-params.scala

@@ -0,0 +1,4 @@
+object Test {
+  def +[T](x: T): String = "x"
+  +[Int](6): String // error: expression expected but '[' found
+}

tests/pos/unary-with-type-params.scala

@@ -0,0 +1,6 @@
+object Test {
+  def +[T](x: T): String = "x"
+  `+`[Int](6): String // Parser can treat + as identifier when backquoted and followed by a type argument
+  `+`(6): String // Parser can treat + as identifier when backquoted and followed by a parenthesized argument
+  +(6): Int // Parser prioritizes + as unary when possible
+}