Allow change of symbols in WithFixedSym#newLikeThis

The symbol might be overridden, in which case the overriding
symbol should be chosen.

This was observered when trying to pickle test all of ast.
Compiling Trees and untpd together gave discrepancies involving
`untpd.dotty$tools$dotc$ast$Trees$$Instance$$T`. It was
found that the symbol referred to by that type refers
to the abstract type in Trees.Instance, not the alias in
untpd. The bad type was produced by an asSeenFrom of the
`WithFixedSym` type
`Instance.this.untpd.dotty$tools$dotc$ast$Trees$$Instance$$T`
as seen from `untpd`. The as seen from did not pick the new
instance in `untpd`.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index ea4101558..9d692f39f 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -1506,8 +1506,11 @@ object Types {
     override def withSym(sym: Symbol, signature: Signature)(implicit ctx: Context): ThisType =
       unsupported("withSym")
 
-    override def newLikeThis(prefix: Type)(implicit ctx: Context): NamedType =
-      NamedType.withFixedSym(prefix, fixedSym)
+    override def newLikeThis(prefix: Type)(implicit ctx: Context): NamedType = {
+      var newSym = prefix.member(fixedSym.name).symbol
+      if (!newSym.exists) newSym = fixedSym
+      NamedType.withFixedSym(prefix, newSym)
+    }
 
     override def equals(that: Any) = that match {
       case that: WithFixedSym => this.prefix == that.prefix && (this.fixedSym eq that.fixedSym)