Merge pull request scala#3331 from retronym/ticket/8120

SI-8120 Avoid tree sharing when typechecking patmat anon functions

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4240,7 +4240,11 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
               }
             val ids = for (p <- params) yield Ident(p.name)
             val selector1 = atPos(tree.pos.focusStart) { if (arity == 1) ids.head else gen.mkTuple(ids) }
-            val body = treeCopy.Match(tree, selector1, cases)
+            // SI-8120 If we don't duplicate the cases, the original Match node will share trees with ones that
+            //         receive symbols owned by this function. However if, after a silent mode session, we discard
+            //         this Function and try a different approach (e.g. applying a view to the reciever) we end up
+            //         with orphaned symbols which blows up far down the pipeline (or can be detected with -Ycheck:typer).
+            val body = treeCopy.Match(tree, selector1, (cases map duplicateAndKeepPositions).asInstanceOf[List[CaseDef]])
             typed1(atPos(tree.pos) { Function(params, body) }, mode, pt)
           }
         } else

test/files/pos/t8120.scala

@@ -0,0 +1,9 @@
+object A {
+  class C {
+    def m(a: Nothing): Int = 0
+  }
+  implicit class RichAny(a: Any) {
+    def m(a: Any): Int = 0
+  }
+  (new C).m({ case (x, y) => x } : Any => Any)
+}