Harden typedUnaply

In the tested case, an unApply ended up with an inserted toplevel apply method,
which confused the follow-on logic. This case is now rejected.

Author: odersky

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

@@ -879,19 +879,25 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
      */
     def trySelectUnapply(qual: untpd.Tree)(fallBack: Tree => Tree): Tree = {
       // try first for non-overloaded, then for overloaded ocurrences
-      def tryWithName(name: TermName)(fallBack: Tree => Tree)(implicit ctx: Context): Tree =
-        tryEither { implicit ctx =>
-          val specificProto = new UnapplyFunProto(selType, this)
-          typedExpr(untpd.Select(qual, name), specificProto)
+      def tryWithName(name: TermName)(fallBack: Tree => Tree)(implicit ctx: Context): Tree = {
+        def tryWithProto(pt: Type)(implicit ctx: Context) = {
+          val result = typedExpr(untpd.Select(qual, name), new UnapplyFunProto(pt, this))
+          if (!result.symbol.exists || result.symbol.name == name) result
+          else notAnExtractor(result)
+          	// It might be that the result of typedExpr is an `apply` selection or implicit conversion.
+          	// Reject in this case.
+        }
+        tryEither {
+          implicit ctx => tryWithProto(selType)
         } {
           (sel, _) =>
-            tryEither { implicit ctx =>
-              val genericProto = new UnapplyFunProto(WildcardType, this)
-              typedExpr(untpd.Select(qual, name), genericProto)
+            tryEither {
+              implicit ctx => tryWithProto(WildcardType)
             } {
               (_, _) => fallBack(sel)
             }
         }
+      }
       // try first for unapply, then for unapplySeq
       tryWithName(nme.unapply) {
         sel => tryWithName(nme.unapplySeq)(_ => fallBack(sel)) // for backwards compatibility; will be dropped

tests/neg/parser-stability-20.scala

@@ -0,0 +1,5 @@
+object x0 {
+def unapply= Array
+x0 match
+x0        // error
+case x0(  // error // error