More inclusive isStable test.

(1) Refinements of stable types are stable
(2) TypeVars instantiated to stable types are stable.

Author: odersky

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

@@ -89,9 +89,10 @@ object Types {
     final def isValueType: Boolean = this.isInstanceOf[ValueType]
 
     /** Does this type denote a stable reference (i.e. singleton type)? */
-    final def isStable(implicit ctx: Context): Boolean = this match {
+    final def isStable(implicit ctx: Context): Boolean = stripTypeVar match {
       case tp: TermRef => tp.termSymbol.isStable && tp.prefix.isStable
       case _: SingletonType => true
+      case tp: RefinedType => tp.parent.isStable
       case NoPrefix => true
       case _ => false
     }