patmat-populated.scala

trait Elems {
  class Elem[A] extends Dummy

  val UnitElement: Elem[Unit]

  // Workaround 1: if you use an object rather than a val, the typechecker
  // uses GADT refinement to adapt the patten type of `a.Elem[Unit]` to
  // `a.Elem[$1]`. See `inferModulePattern`, which calls `instantiateTypeParams`.
  //
  // object UnitElement extends Elem[Unit]

  // Workaround 2: Define `Dummy` at the top level, rather than
  // as a member of `Elems`. Even without GADT refinement, the
  // check for `isPopulated(BadMatch.this.a.Elem[Unit], ?#Elem[?]`)
  // seems to be happy that there is a value that inhabits both
  // types.
  //
  // I think thie problem here is that `?#Elem[?]`.baseType(DummyClass)
  // returns `Elems.this.Dummy`, rather than `?#Dummy`. (? is the
  // Wildcard type).
  //
  // We should update `AsSeenFromMap` as follows to fix this:
  //
  //   diff --git a/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala b/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
  // index af51281..316db7a 100644
  // --- a/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
  // +++ b/src/reflect/scala/reflect/internal/tpe/TypeMaps.scala
  // @@ -616,3 +616,3 @@ private[internal] trait TypeMaps {
  //      private def matchesPrefixAndClass(pre: Type, clazz: Symbol)(candidate: Symbol) =
  // -      (clazz == candidate) && (pre.widen.typeSymbol isSubClass clazz)
  // +      (clazz == candidate) && ((pre.widen.typeSymbol isSubClass clazz) || (pre eq WildcardType))

  trait Dummy


}

class BadMatch[A <: Elems](a: A) {
  def toLuaValue(eX: a.Elem[_]): String = eX match {
    case a.UnitElement => "" // type mismatch
  }
}