Merge pull request #6031 from dotty-staging/fix-#6003

Test for #6003

Author: liufengyun

tests/pos/i6003.scala

@@ -0,0 +1,12 @@
+object Test {
+  opaque type T = String
+  object T {
+    def unwrap(t: T): String = t
+  }
+
+  opaque type U = String
+  type W = U
+  object U {
+    def unwrap(w: W): String = w: U
+  }
+}