Type constructors of kind (*, *) -> * don't infer properly

[LukaJCB]

Minimized code

class Foo[T[_, _], F[_], A, B](val fa: T[F[A], F[B]]) extends AnyVal

def x[T[_, _]](tmab: T[Either[Int, String], Either[Int, Int]]) = 
  new Foo(tmab)

Output

Found:    (tmab : T[Either[Int, String], Either[Int, Int]])
Required: T²[F[A], F[B]]

where:    A  is a type variable
          B  is a type variable
          F  is a type variable with constraint <: [_$14] =>> Any
          T  is a type in method x with bounds <: [_$15, _$16] =>> Any
          T² is a type variable with constraint <: [_$12, _$13] =>> Any

Expectation

This should compile fine and infer F as Either[Int, *], it compiles fine in Scala 2.

The larger context here is that in Cats we have a Bitraverse typeclass:

trait Bitraverse[F[_, _]] {
  def bisequence[G[_]: Applicative, A, B](fg: F[G[A], G[B]]): G[F[A, B]]
}

And the Ops for it stopped working in Dotty:

implicit class BitraverseOps[T[_, _], M[_], A, B](private val tmamb: T[M[A], M[B]]) extends AnyVal {
  def bisequence(implicit T: Bitraverse[T], P: Applicative[M]): M[T[A, B]] =
    T.bisequence(tmamb)
}

def x[T[_, _]: Bitraverse, B](tmab: T[Either[Int, String], Either[Int, Int]]) = 
  tmab.bisequence

[error] value bisequence is not a member of T[Either[Int, String], Either[Int, Int]], but could be made available as an extension method.

The following import might make progress towards fixing the problem:

  import collection.Searching.search



where:    T is a type in method x with bounds <: [_$9, _$10] =>> Any