Infinfite loop when returning anonymous higher kinded type

[mwisnicki]

minimized code

type X[T] = ({ type F[_]; type R = F[T]})#R

expectation

This compiles in Scala 2.13 but fail in latest dotty with Recursion limit exceeded.

context

While it doesn't matter for this bug what I'm actually trying to define is

trait Monad[F[_]]
type of[M[_[_]], T] = ({ type F[_]; type R = (given M[F]) => F[T]})#R
def foo(a: Monad of String) = ???

[odersky]

The problem is that structural types with type members are allowed only as an afterthought. I believe neither scalac nor dotc implements them properly, and, furthermore, the implementation schemes are completely different. If I lift out the type members into a base class, the code compiles:

class C[T] {
  type F[_]; type R
}

type X[T] = (C[T] { type F[_]; type R = F[T]})#R

Would such a lifting be acceptable?