Dealias before type erasing

... and likewise for taking a signature. The previous case worked
in all cases except when faced with an alias like `type Id[T] = T`.
In that case, it would disregard the argument and erase to Object.

Author: odersky

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

@@ -355,7 +355,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
    *   - For NoType or NoPrefix, the type itself.
    *   - For any other type, exception.
    */
-  private def apply(tp: Type)(implicit ctx: Context): Type = tp match {
+  private def apply(tp: Type)(implicit ctx: Context): Type = tp.dealias match {
     case _: ErasedValueType =>
       tp
     case tp: TypeRef =>
@@ -487,7 +487,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
    *  Need to ensure correspondence with erasure!
    */
   private def sigName(tp: Type)(implicit ctx: Context): TypeName = try {
-    tp match {
+    tp.dealias match {
       case ErasedValueType(_, underlying) =>
         sigName(underlying)
       case tp: TypeRef =>