Fix #4369: Add regression test

Author: nicolasstucki

tests/neg/i4369.scala

@@ -0,0 +1,4 @@
+trait X[R <: Z, Z >: X[R, R]] {
+   def foo(x: X[R, R]): Z = x // error: Found: X[R, R](x)    Required: Z
+}
+class Z extends X[Z, Z]

tests/neg/i4369b.scala

@@ -0,0 +1,4 @@
+trait X[R <: Z, Z >: X[R, R] <: X[R, R]] {
+  implicitly[Z =:= X[R, R]] // error:  Cannot prove that Z =:= X[R, R]
+}
+class Z extends X[Z, Z]

tests/neg/i4369c.scala

@@ -0,0 +1,5 @@
+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 } // error