Union types don't get widened within a type lambda

[LukaJCB]

Minimized code

trait Applicative[F[_]]

def traverse[F[_]: Applicative, A, B](as: List[A])(f: A => F[B]) = ???

implicit def eitherApplicative[A]: Applicative[[X] =>> Either[A, X]] = ???

traverse(List(1, 2))(i => if (true) Right(i) else Left(i))

Output

no implicit argument of type Applicative[([X0] =>> Right[Nothing, X0] | Left[Int, X0])] was found for an implicit parameter of method traverse

Expectation

This should compile fine as it does in Scala 2.