Merge pull request #6870 from dotty-staging/fix-#4369

Fix #4369: Add regression tests

Author: liufengyun

tests/neg/i4369a.scala

@@ -0,0 +1,2 @@
+trait X[R <: Z, Z >: X[R, R]] // error
+class Z extends X[Z, Z]

tests/neg/i4369b.scala

@@ -0,0 +1,2 @@
+trait X[R <: Z, Z >: X[R, R] <: X[R, R]] // error // error
+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 } // error // error // error
+}
+class Foo extends X { type R = Foo; type Z = Foo }