Pattern Match Abstract Type Trait Member

Question

sealed trait Foo {
   type T <: Option[Any]
   val x : T
}

case class Bar(x : Option[Int]) extends Foo { 
   type T = Option[Int]
}

val baz : Foo = Bar(Some(42))

baz.x match {
   case Some(a) => a
   case None => 1337
}

This is the error message when attempting to pattern match:

:12: error: pattern type is incompatible with expected type;
found   : None.type
required: baz.T

I believe this is due to type-erasure on type T.

Answer 1

I'm not sure why this fails, probably a limitation in the type inference. The following works:

(baz.x: Option[Any]) match {
   case Some(a) => a
   case None => 1337
}

In any case, asking for a subtype of Option does not make sense. Better define the option's element type:

sealed trait Foo {
  type A
  def x: Option[A]
}

case class Bar(x : Option[Int]) extends Foo { 
  type A = Int
}

val baz : Foo = Bar(Some(42))

baz.x match {
  case Some(a) => a
  case None    => 1337
}