Fix iso

Author: odersky

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

@@ -79,6 +79,13 @@ object TypeErasure {
   abstract case class ErasedValueType(tycon: TypeRef, erasedUnderlying: Type)
   extends CachedGroundType with ValueType {
     override def computeHash(bs: Hashable.Binders) = doHash(bs, tycon, erasedUnderlying)
+
+    override def iso(that: Any, bs: Hashable.BinderPairs) = that match {
+      case that: ErasedValueType =>
+        tycon.equals(that.tycon, bs) && erasedUnderlying.equals(that.erasedUnderlying)
+      case _ =>
+        false
+    }
   }
 
   final class CachedErasedValueType(tycon: TypeRef, erasedUnderlying: Type)