Fix #4369: Add regression test

[nicolasstucki]

No description provided.

[anatoliykmetyuk]

Are you sure #4369 is fixed? IMO the following shouldn't compile even with the method commented:

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

From the issue:

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.