SI-7571 Allow nesting of anonymous classes in value classes

5d9cde105e added deep prohibition of nested classes within
a value class. This has the undesirable side effect of
prohibiting partial functions literals in method bodies
of a value class.

The intention of that prohibition was to avoid problems
in code using Type Tests, such as:

    class C(val inner: A) extends AnyVal {
      class D
    }
    def foo(a: Any, other: C) = a match { case _ : other.D }

Here, the pattern usually checks that `a.$outer == other`.
But that is incongruent with the way that `other` is erased
to `A`.

However, not all nested classes could lead us into this trap.
This commit slightly relaxes the restriction to allow anonymous
classes, which can't appear in a type test.

The test shows that the translation generates working code.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 29cd3d4bfa..ed2963fb0f 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -1455,8 +1455,8 @@ trait Typers extends Modes with Adaptations with Tags {
               implRestriction(tree, "nested object")
             //see https://issues.scala-lang.org/browse/SI-6444
             //see https://issues.scala-lang.org/browse/SI-6463
-            case _: ClassDef =>
-              implRestriction(tree, "nested class")
+            case cd: ClassDef if !cd.symbol.isAnonymousClass => // Don't warn about partial functions, etc. SI-7571
+              implRestriction(tree, "nested class") // avoiding Type Tests that might check the $outer pointer.
             case Select(sup @ Super(qual, mix), selector) if selector != nme.CONSTRUCTOR && qual.symbol == clazz && mix != tpnme.EMPTY =>
               //see https://issues.scala-lang.org/browse/SI-6483
               implRestriction(sup, "qualified super reference")