Wrong lower type bound is accepted

[sir-wabbit]

trait X[R <: Z, Z >: X[R, R]] {
  // def foo(x: X[R, R]): Z = x
}
class Z extends X[Z, Z]

Somehow this compiles, even though foo doesn't. Note that class Z is not a super type of X[Z, Z], which is required by the second type parameter of trait X.

trait X[R <: Z, Z >: X[R, R] <: X[R, R]] {
  // implicitly[Z =:= X[R, R]] // won't compile
}
class Z extends X[Z, Z]

[sir-wabbit]

This might be related to #4368 since encoding the above snippet using type members:

trait X { self =>
  type R <: Z
  type Z >: X { type R = self.R; type Z = self.R }
}
class Foo extends X { type R = Foo; type Z = Foo }

causes stack overflow.