Can't prove equality between syntactically equal type aliases to an existential type

[sir-wabbit]

trait R[Y] { type T }
type A = a.T forSome { val a: R[Y]; type Y }
type B = a.T forSome { val a: R[Y]; type Y }
final case class Equal[A, B >: A <: A]()
Equal[A, B] // doesn't compile
implicitly[A =:= B] // doesn't compile

Tested in 2.11 on ScalaFiddle and 2.12.1 in REPL.

[milessabin]

There is a problem, but I don't think it's quite what the headline describes. For instance, the following, which I think is equivalent to what you intended above, is just fine,

scala> type A = a.T forSome { val a: R[_]}
defined type alias A

scala> type B = a.T forSome { val a: R[_]}
defined type alias B

scala> implicitly[A =:= B]
res13: A =:= B = <function1>

[sir-wabbit]

This might be related:

val x = "a"
type X = a.type forSome { val a: x.type; type X }
implicitly[X =:= x.type]

compiles

val x = "a"
type X = a.type forSome { val a: x.type; type X >: a.type <: a.type }
implicitly[X =:= x.type]

doesn't