Merge pull request #6441 from dotty-staging/fix-#4509

Fix #4509: Eta-expand as erased contextual

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1159,9 +1159,10 @@ object desugar {
     Function(param :: Nil, Block(vdefs, body))
   }
 
-  def makeContextualFunction(formals: List[Type], body: Tree)(implicit ctx: Context): Tree = {
-    val params = makeImplicitParameters(formals.map(TypeTree), Given)
-    new FunctionWithMods(params, body, Modifiers(Implicit | Given))
+  def makeContextualFunction(formals: List[Type], body: Tree, isErased: Boolean)(implicit ctx: Context): Tree = {
+    val mods = if (isErased) Given | Erased else Given
+    val params = makeImplicitParameters(formals.map(TypeTree), mods)
+    new FunctionWithMods(params, body, Modifiers(Implicit | mods))
   }
 
   /** Add annotation to tree:

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -232,7 +232,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
           case dummyTreeOfType(tp) :: Nil if !(tp isRef defn.NullClass) => "null: " ~ toText(tp)
           case _ => toTextGlobal(args, ", ")
         }
-        return "[applied to " ~ (Str("given ") provided tp.isContextual) ~ "(" ~ argsText ~ ") returning " ~ toText(resultType) ~ "]"
+        return "[applied to " ~ (Str("given ") provided tp.isContextual) ~ (Str("erased ") provided tp.isErasedMethod) ~ "(" ~ argsText ~ ") returning " ~ toText(resultType) ~ "]"
       case IgnoredProto(ignored) =>
         return "?" ~ (("(ignored: " ~ toText(ignored) ~ ")") provided ctx.settings.verbose.value)
       case tp @ PolyProto(targs, resType) =>
@@ -529,10 +529,12 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
       case Function(args, body) =>
         var implicitSeen: Boolean = false
         var contextual: Boolean = false
+        var isErased: Boolean = false
         def argToText(arg: Tree) = arg match {
           case arg @ ValDef(name, tpt, _) =>
             val implicitText =
               if ((arg.mods is Given)) { contextual = true; "" }
+              else if ((arg.mods is Erased)) { isErased = true; "" }
               else if ((arg.mods is Implicit) && !implicitSeen) { implicitSeen = true; keywordStr("implicit ") }
               else ""
             implicitText ~ toText(name) ~ optAscription(tpt)
@@ -545,6 +547,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         }
         changePrec(GlobalPrec) {
 		  (keywordText("given ") provided contextual) ~
+		  (keywordText("erased ") provided isErased) ~
           argsText ~ " => " ~ toText(body)
         }
       case InfixOp(l, op, r) =>

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2184,7 +2184,7 @@ class Typer extends Namer
 
   protected def makeContextualFunction(tree: untpd.Tree, pt: Type)(implicit ctx: Context): Tree = {
     val defn.FunctionOf(formals, _, true, _) = pt.dropDependentRefinement
-    val ifun = desugar.makeContextualFunction(formals, tree)
+    val ifun = desugar.makeContextualFunction(formals, tree, defn.isErasedFunctionType(pt))
     typr.println(i"make contextual function $tree / $pt ---> $ifun")
     typed(ifun, pt)
   }

tests/pos/i4509.scala

@@ -0,0 +1,4 @@
+object Main {
+  def fun[T](op: given erased (Int) => T) = op given 0
+  fun { }
+}