SI-4897 derive expected value from single type

when the type in a type test is, say, `C.this.A.type`,
must use the corresponding term `C.this.A` to test for equality
if you use the naive REF(<C.this.A.type>.symbol), you'll
get a path like `OwnerOfA.this.A`,
where `OwnerOfA` might be a superclass of `C`, and
explicitouter won't like that

Author: Adriaan Moors

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

@@ -1024,7 +1024,7 @@ trait PatternMatching extends Transform with TypingTransformers with ast.TreeDSL
         else expectedTp match {
           // TODO: [SPEC] the spec requires `eq` instead of `==` for singleton types
           // this implies sym.isStable
-          case SingleType(_, sym)                       => and(equalsTest(CODE.REF(sym), testedBinder), typeTest(testedBinder, expectedTp.widen))
+          case SingleType(_, sym)                       => and(equalsTest(gen.mkAttributedQualifier(expectedTp), testedBinder), typeTest(testedBinder, expectedTp.widen))
           // must use == to support e.g. List() == Nil
           case ThisType(sym) if sym.isModule            => and(equalsTest(CODE.REF(sym), testedBinder), typeTest(testedBinder, expectedTp.widen))
           case ConstantType(Constant(null)) if testedBinder.info.widen <:< AnyRefClass.tpe

test/files/run/t4897.check

@@ -0,0 +1 @@
+joepie

test/files/run/t4897.scala

@@ -0,0 +1,10 @@
+class CSuper {
+  object A
+}
+class C extends CSuper {
+  def f = (A: AnyRef) match { case _: A.type  => "joepie" }
+}
+
+object Test extends C with App {
+  println(f)
+}