Fix #6225: add test

Author: liufengyun

tests/neg/i6225.scala

@@ -0,0 +1,23 @@
+object O1 {
+  type A[X] = X
+  opaque type T = A
+}
+
+object O2 {
+  opaque type A[X] = X
+  object A {
+    opaque type T = A
+  }
+}
+
+object O3 {
+  opaque type R[X] = R[X]   // error
+}
+
+object O4{
+  opaque type T[X] = Any
+}
+
+object O5{
+  opaque type T[X] = Nothing
+}