SI-7285 Fix match analysis with nested objects.

The fix for SI-6146 introduced `nestedMemberType` to
enumerate sealed subtypes based on the (prefixed) type
of the scrutinee and the symbols of its sealed subclasses.
That method needed to widen `ThisType(modSym)`s to
`ModuleTypeRef(modSym)` before calling `asSeenFrom`.

However, this could lead to confused in the match analysis,
which sees `ModuleTypeRef` as distinct from singleton types
on the same modules (after all, they aren't =:=). Spurious
warnings ensued.

This commit makes two changes:

 - conditionally re-narrow the result of `asSeenFrom` in `nestedMemberType`.
 - present `a.b.SomeModule.type` as `SomeModule` in warnings emitted
   by the pattern matcher.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index a27b37dae5..a2c9f1fadf 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -5036,10 +5036,11 @@ trait Types extends api.Types { self: SymbolTable =>
       if (tp.isTrivial) tp
       else if (tp.prefix.typeSymbol isNonBottomSubClass owner) {
         val widened = tp match {
-          case _: ConstantType => tp        // Java enum constants: don't widen to the enum type!
-          case _               => tp.widen  // C.X.type widens to C.this.X.type, otherwise `tp asSeenFrom (pre, C)` has no effect.
+          case _: ConstantType => tp // Java enum constants: don't widen to the enum type!
+          case _               => tp.widen // C.X.type widens to C.this.X.type, otherwise `tp asSeenFrom (pre, C)` has no effect.
         }
-        widened asSeenFrom (pre, tp.typeSymbol.owner)
+        val memType = widened asSeenFrom (pre, tp.typeSymbol.owner)
+        if (tp eq widened) memType else memType.narrow
       }
       else loop(tp.prefix) memberType tp.typeSymbol