isDerivedValueClass: try to avoid causing cyclic references

This fixes the problem showcased in #412.

Author: smarter

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

@@ -13,10 +13,15 @@ import StdNames._
 object ValueClasses {
 
   def isDerivedValueClass(d: SymDenotation)(implicit ctx: Context) =
-    d.isClass &&
-      d.derivesFrom(defn.AnyValClass) &&
-      (d.symbol ne defn.AnyValClass) &&
-      !d.isPrimitiveValueClass
+    d.initial match {
+      case di: ClassDenotation =>
+        d.isClass &&
+        di.derivesFrom(defn.AnyValClass)(ctx.withPhase(di.validFor.firstPhaseId)) &&
+        (d.symbol ne defn.AnyValClass) &&
+        !d.isPrimitiveValueClass
+      case _ =>
+        false
+    }
 
   def isMethodWithExtension(d: SymDenotation)(implicit ctx: Context) =
     d.isSourceMethod &&