Improve implicit conversion docs and some more tests

docs/docs/reference/contextual/conversions.md

@@ -14,6 +14,10 @@ implied for Conversion[String, Token] {
   def apply(str: String): Token = new KeyWord(str)
 }
 ```
+Using an implied alias this can be expressed more concisely as:
+```scala
+implied for Conversion[String, Token] = new KeyWord(_)
+```
 An implicit conversion is applied automatically by the compiler in three situations:
 
 1. If an expression `e` has type `T`, and `T` does not conform to the expression's expected type `S`.
@@ -33,9 +37,8 @@ If such an instance `C` is found, the expression `e` is replaced by `C.apply(e)`
 primitive number types to subclasses of `java.lang.Number`. For instance, the
 conversion from `Int` to `java.lang.Integer` can be defined as follows:
 ```scala
-implied int2Integer for Conversion[Int, java.lang.Integer] {
-  def apply(x: Int) = new java.lang.Integer(x)
-}
+implied int2Integer for Conversion[Int, java.lang.Integer] =
+  new java.lang.Integer(x)
 ```
 
 2. The "magnet" pattern is sometimes used to express many variants of a method. Instead of defining overloaded versions of the method, one can also let the method take one or more arguments of specially defined "magnet" types, into which various argument types can be converted. E.g.
@@ -56,15 +59,9 @@ object Completions {
     //
     //   CompletionArg.fromStatusCode(statusCode)
 
-    implied fromString for Conversion[String, CompletionArg] {
-      def apply(s: String) = CompletionArg.Error(s)
-    }
-    implied fromFuture for Conversion[Future[HttpResponse], CompletionArg] {
-      def apply(f: Future[HttpResponse]) = CompletionArg.Response(f)
-    }
-    implied fromStatusCode for Conversion[Future[StatusCode], CompletionArg] {
-      def apply(code: Future[StatusCode]) = CompletionArg.Status(code)
-    }
+    implied fromString for Conversion[String, CompletionArg] = Error(_)
+    implied fromFuture for Conversion[Future[HttpResponse], CompletionArg] = Response(_)
+    implied fromStatusCode for Conversion[Future[StatusCode], CompletionArg] = Status(_)
   }
   import CompletionArg._
 

tests/pos-with-compiler/DecoratorsX.scala

@@ -0,0 +1,205 @@
+package dotty.tools.dotc
+package core
+
+import annotation.tailrec
+import Symbols._
+import Contexts._, Names._, Phases._, printing.Texts._, printing.Printer
+import util.Spans.Span, util.SourcePosition
+import collection.mutable.ListBuffer
+import dotty.tools.dotc.transform.MegaPhase
+import ast.tpd._
+import scala.language.implicitConversions
+import printing.Formatting._
+
+/** This object provides useful conversions and extension methods for types defined elsewhere */
+object DecoratorsX {
+
+  /** Turns Strings into PreNames, adding toType/TermName methods */
+  class StringPreName(s: String) extends AnyVal with PreName {
+    def toTypeName: TypeName = typeName(s)
+    def toTermName: TermName = termName(s)
+    def toText(printer: Printer): Text = Str(s)
+  }
+  implied for Conversion[String, StringPreName] = new StringPreName(_)
+
+  final val MaxFilterRecursions = 1000
+
+  implied {
+    def (s: String) splitWhere (f: Char => Boolean, doDropIndex: Boolean): Option[(String, String)] = {
+      def splitAt(idx: Int, doDropIndex: Boolean): Option[(String, String)] =
+        if (idx == -1) None
+        else Some((s.take(idx), s.drop(if (doDropIndex) idx + 1 else idx)))
+
+      splitAt(s.indexWhere(f), doDropIndex)
+    }
+
+    /** Implements a findSymbol method on iterators of Symbols that
+     *  works like find but avoids Option, replacing None with NoSymbol.
+     */
+    def (it: Iterator[Symbol]) findSymbol(p: Symbol => Boolean): Symbol = {
+      while (it.hasNext) {
+        val sym = it.next()
+        if (p(sym)) return sym
+      }
+      NoSymbol
+    }
+
+    def (xs: List[T]) mapconserve [T, U] (f: T => U): List[U] = {
+      @tailrec
+      def loop(mapped: ListBuffer[U], unchanged: List[U], pending: List[T]): List[U] =
+        if (pending.isEmpty) {
+          if (mapped eq null) unchanged
+          else mapped.prependToList(unchanged)
+        } else {
+          val head0 = pending.head
+          val head1 = f(head0)
+
+          if (head1.asInstanceOf[AnyRef] eq head0.asInstanceOf[AnyRef])
+            loop(mapped, unchanged, pending.tail)
+          else {
+            val b = if (mapped eq null) new ListBuffer[U] else mapped
+            var xc = unchanged
+            while (xc ne pending) {
+              b += xc.head
+              xc = xc.tail
+            }
+            b += head1
+            val tail0 = pending.tail
+            loop(b, tail0.asInstanceOf[List[U]], tail0)
+          }
+        }
+      loop(null, xs.asInstanceOf[List[U]], xs)
+    }
+
+    /** Like `xs filter p` but returns list `xs` itself  - instead of a copy -
+     *  if `p` is true for all elements and `xs` is not longer
+     *  than `MaxFilterRecursions`.
+     */
+    def (xs: List[T]) filterConserve [T] (p: T => Boolean): List[T] = {
+      def loop(xs: List[T], nrec: Int): List[T] = xs match {
+        case Nil => xs
+        case x :: xs1 =>
+          if (nrec < MaxFilterRecursions) {
+            val ys1 = loop(xs1, nrec + 1)
+            if (p(x))
+              if (ys1 eq xs1) xs else x :: ys1
+            else
+              ys1
+          } else xs filter p
+      }
+      loop(xs, 0)
+    }
+
+    /** Like `(xs, ys).zipped.map(f)`, but returns list `xs` itself
+     *  - instead of a copy - if function `f` maps all elements of
+     *  `xs` to themselves. Also, it is required that `ys` is at least
+     *  as long as `xs`.
+     */
+    def (xs: List[T]) zipWithConserve [T, U] (ys: List[U])(f: (T, U) => T): List[T] =
+      if (xs.isEmpty || ys.isEmpty) Nil
+      else {
+        val x1 = f(xs.head, ys.head)
+        val xs1 = xs.tail.zipWithConserve(ys.tail)(f)
+        if ((x1.asInstanceOf[AnyRef] eq xs.head.asInstanceOf[AnyRef]) &&
+            (xs1 eq xs.tail)) xs
+        else x1 :: xs1
+      }
+
+    def (xs: List[T]) hasSameLengthAs [T, U] (ys: List[U]): Boolean = {
+      @tailrec def loop(xs: List[T], ys: List[U]): Boolean =
+        if (xs.isEmpty) ys.isEmpty
+        else ys.nonEmpty && loop(xs.tail, ys.tail)
+      loop(xs, ys)
+    }
+
+    @tailrec
+    def (xs: List[T]) eqElements [T] (ys: List[AnyRef]): Boolean = xs match {
+      case x :: _ =>
+        ys match {
+          case y :: _ =>
+            x.asInstanceOf[AnyRef].eq(y) &&
+            xs.tail.eqElements(ys.tail)
+          case _ => false
+        }
+      case nil => ys.isEmpty
+    }
+
+    def (xs: List[T]) | [T] (ys: List[T]): List[T] = xs ::: (ys filterNot (xs contains _))
+
+    /** Intersection on lists seen as sets */
+    def (xs: List[T]) & [T] (ys: List[T]): List[T] = xs filter (ys contains _)
+
+    def (xss: List[List[T]]) nestedMap [T, U] (f: T => U): List[List[U]] = xss map (_ map f)
+    def (xss: List[List[T]]) nestedMapconserve [T, U](f: T => U): List[List[U]] = xss mapconserve (_ mapconserve f)
+
+    def (text: Text) show given (ctx: Context): String =
+      text.mkString(ctx.settings.pageWidth.value, ctx.settings.printLines.value)
+
+    /** Test whether a list of strings representing phases contains
+    *  a given phase. See [[config.CompilerCommand#explainAdvanced]] for the
+    *  exact meaning of "contains" here.
+    */
+    def (names: List[String]) containsPhase (phase: Phase): Boolean =
+      names.nonEmpty && {
+        phase match {
+          case phase: MegaPhase => phase.miniPhases.exists(names.containsPhase(_))
+          case _ =>
+            names exists { name =>
+              name == "all" || {
+                val strippedName = name.stripSuffix("+")
+                val logNextPhase = name != strippedName
+                phase.phaseName.startsWith(strippedName) ||
+                  (logNextPhase && phase.prev.phaseName.startsWith(strippedName))
+              }
+            }
+        }
+      }
+
+    def (x: T) reporting [T] (op: T => String, printer: config.Printers.Printer = config.Printers.default): T = {
+      printer.println(op(x))
+      x
+    }
+
+    def (x: T) assertingErrorsReported [T] given Context: T = {
+      assert(the[Context].reporter.errorsReported)
+      x
+    }
+
+    def (x: T) assertingErrorsReported [T] (msg: => String) given Context: T = {
+      assert(the[Context].reporter.errorsReported, msg)
+      x
+    }
+
+    /** General purpose string formatting */
+    def (sc: StringContext) i (args: Any*) given Context: String =
+      new StringFormatter(sc).assemble(args)
+
+    /** Formatting for error messages: Like `i` but suppress follow-on
+     *  error messages after the first one if some of their arguments are "non-sensical".
+     */
+    def (sc: StringContext) em (args: Any*) given Context: String =
+      new ErrorMessageFormatter(sc).assemble(args)
+
+    /** Formatting with added explanations: Like `em`, but add explanations to
+     *  give more info about type variables and to disambiguate where needed.
+     */
+    def (sc: StringContext) ex (args: Any*) given Context: String =
+      explained(sc.em(args: _*))
+
+    /** Formatter that adds syntax highlighting to all interpolated values */
+    def (sc: StringContext) hl (args: Any*) given Context: String =
+      new SyntaxFormatter(sc).assemble(args).stripMargin
+
+    def (arr: Array[T]) binarySearch [T <: AnyRef] (x: T): Int =
+      java.util.Arrays.binarySearch(arr.asInstanceOf[Array[Object]], x)
+  }
+
+    /** Entrypoint for explanation string interpolator:
+    *
+    * ```
+    * ex"disambiguate $tpe1 and $tpe2"
+    * ```
+    */
+  def explained(op: given Context => String) given Context: String =
+    printing.Formatting.explained(ctx => op given ctx)
+}

tests/pos/i5966.scala

@@ -0,0 +1,8 @@
+object Test {
+  def foo = given (v: Int) => (x: Int) => v + x
+  implied myInt for Int = 4
+
+  foo.apply(1)
+  foo given 2
+  foo(3)
+}

tests/pos/reference/instances.scala

@@ -274,15 +274,9 @@ object Completions {
     //
     //   CompletionArg.from(statusCode)
 
-    implied fromString for Conversion[String, CompletionArg] {
-      def apply(s: String) = CompletionArg.Error(s)
-    }
-    implied fromFuture for Conversion[Future[HttpResponse], CompletionArg] {
-      def apply(f: Future[HttpResponse]) = CompletionArg.Response(f)
-    }
-    implied fromStatusCode for Conversion[Future[StatusCode], CompletionArg] {
-      def apply(code: Future[StatusCode]) = CompletionArg.Status(code)
-    }
+    implied fromString for Conversion[String, CompletionArg] = Error(_)
+    implied fromFuture for Conversion[Future[HttpResponse], CompletionArg] = Response(_)
+    implied fromStatusCode for Conversion[Future[StatusCode], CompletionArg] = Status(_)
   }
   import CompletionArg._
 

tests/run/cochis-example.scala

@@ -0,0 +1,14 @@
+
+import Predef.{$conforms => _}
+trait A {
+  implied id[X] for (X => X) = x => x
+  def trans[X](x: X) given (f: X => X) = f(x)  // (2)
+}
+object Test extends A with App{
+  implied succ for (Int => Int) = x => x + 1     // (3)
+  def bad[X](x: X): X = trans[X](x)              // (4) unstable definition !
+  val v1 = bad [Int] (3)                         // (5) evaluates to 3
+  assert(v1 == 3)
+  val v2 = trans [Int] (3)
+  assert(v2 == 4)
+}