fix #1642: disallow value classe wrapping value class

Author: liufengyun

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

@@ -427,7 +427,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
   private def eraseDerivedValueClassRef(tref: TypeRef)(implicit ctx: Context): Type = {
     val cls = tref.symbol.asClass
     val underlying = underlyingOfValueClass(cls)
-    if (underlying.exists) ErasedValueType(tref, valueErasure(underlying))
+    if (underlying.exists && isDerivedValueClass(underlying.typeSymbol)) ErasedValueType(tref, valueErasure(underlying))
     else NoType
   }
 

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -720,6 +720,9 @@ object RefChecks {
           case List(param) =>
             if (param.is(Mutable))
               ctx.error("value class parameter must not be a var", param.pos)
+
+            if (isDerivedValueClass(param.info.typeSymbol))
+              ctx.error("value class may not wrap another user-defined value class", param.pos)
           case _ =>
             ctx.error("value class needs to have exactly one val parameter", clazz.pos)
         }

tests/neg/i1642.scala

@@ -0,0 +1 @@
+class Test2(val valueVal: Test2) extends AnyVal