Fix #9509: Reveal further arguments in extension method applications

They were hidden in an IgnroedProto before for fear that we might need an implicit
conversion. But none is possible at this point.

Author: odersky

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

@@ -2086,6 +2086,13 @@ trait Applications extends Compatibility {
    *  where <type-args> comes from `pt` if it is a (possibly ignored) PolyProto.
    */
   def extMethodApply(methodRef: untpd.Tree, receiver: Tree, pt: Type)(using Context): Tree = {
+    // (1) always reveal further arguments in extension method applications,
+    // (2) but do not reveal anything else in a prototype.
+    // (1) is needed for i9509.scala (2) is needed for i6900.scala
+    val normPt = pt match
+      case IgnoredProto(ignored: FunProto) => ignored
+      case _ => pt
+
     /** Integrate the type arguments from `currentPt` into `methodRef`, and produce
      *  a matching expected type.
      *  If `currentPt` is ignored, the new expected type will be ignored too.
@@ -2097,9 +2104,9 @@ trait Applications extends Compatibility {
         val core = untpd.TypeApply(methodRef, targs.map(untpd.TypedSplice(_)))
         (core, if (wasIgnored) IgnoredProto(restpe) else restpe)
       case _ =>
-        (methodRef, pt)
+        (methodRef, normPt)
     }
-    val (core, pt1) = integrateTypeArgs(pt)
+    val (core, pt1) = integrateTypeArgs(normPt)
     val app = withMode(Mode.SynthesizeExtMethodReceiver) {
       typed(untpd.Apply(core, untpd.TypedSplice(receiver) :: Nil), pt1, ctx.typerState.ownedVars)
     }

tests/pos/i9509.scala

@@ -0,0 +1,11 @@
+enum AList[+A] {
+  case Cons(head: A, tail: AList[A])
+  case Nil
+}
+
+object AList {
+  extension [A](l: AList[A]) def sum(using numeric: Numeric[A]): A = l match {
+    case Cons(x, xs) => numeric.plus(x, xs.sum(using numeric))
+    case Nil         => numeric.zero
+  }
+}