isDerivedValueClass: try to avoid causing cyclic references

This fixes the problem showcased in #412.

Author: smarter

src/dotty/tools/dotc/transform/ValueClasses.scala

@@ -14,7 +14,7 @@ object ValueClasses {
 
   def isDerivedValueClass(d: SymDenotation)(implicit ctx: Context) =
     d.isClass &&
-      d.derivesFrom(defn.AnyValClass) &&
+      d.derivesFrom(defn.AnyValClass)(ctx.withPhase(d.initial.validFor.phaseId)) &&
       (d.symbol ne defn.AnyValClass) &&
       !d.isPrimitiveValueClass