Clean up logic for isStable

Author: odersky

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

@@ -474,7 +474,10 @@ object Flags {
   assert(AfterLoadFlags.isTermFlags && AfterLoadFlags.isTypeFlags)
 
   /** A value that's unstable unless complemented with a Stable flag */
-  final val UnstableValue = Mutable | Method
+  final val UnstableValue =
+    Mutable | Method | Erased
+      // TODO: Erased should be treated as stable just like final lazy is.
+      // Otherwise the usefulness of Erased is very much reduced.
 
   /** Flags that express the variance of a type parameter. */
   final val VarianceFlags = Covariant | Contravariant

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

@@ -599,8 +599,10 @@ object SymDenotations {
       )
 
     /** Is this a denotation of a stable term (or an arbitrary type)? */
-    final def isStable(implicit ctx: Context) =
-      isType || !is(Erased) && (is(Stable) || !(is(UnstableValue) || info.isInstanceOf[ExprType]))
+    final def isStable(implicit ctx: Context) = {
+      def isUnstableValue = is(UnstableValue) || info.isInstanceOf[ExprType]
+      isType || is(Stable) || !isUnstableValue
+    }
 
     /** Is this a "real" method? A real method is a method which is:
      *  - not an accessor