Never include self types in named parts of a type

According to SLS § 7.2, self types are not a named part of a type,
so they're not part of the implicit scope. Before this commit, this was
usually the case because we normally refer to a class using a TypeRef,
but in some cases a class might appear as a ThisType, and
ThisType#underlying returns the self type, we now use ThisType#tref
instead which just returns a TypeRef corresponding to the class.

Author: smarter

src/dotty/tools/dotc/core/Types.scala

@@ -3450,7 +3450,7 @@ object Types {
           case TypeBounds(_, hi) =>
             apply(x, hi)
           case tp: ThisType =>
-            apply(x, tp.underlying)
+            apply(x, tp.tref)
           case tp: ConstantType =>
             apply(x, tp.underlying)
           case tp: MethodParam =>