Change underlying to superType in TypeEval

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeEval.scala

@@ -18,7 +18,9 @@ object TypeEval:
           // final val one = 1
           // type Two = one.type + one.type
           // ```
-          case tp: TypeProxy if tp.underlying.isStable => tp.underlying.fixForEvaluation
+          case tp: TypeProxy =>
+            val tp1 = tp.superType
+            if tp1.isStable then tp1.fixForEvaluation else tp
           case tp => tp
 
       def constValue(tp: Type): Option[Any] = tp.fixForEvaluation match