Merge pull request #9557 from dotty-staging/fix-#6226

Fix #6226: Add regression test

Author: anatoliykmetyuk

tests/neg/i6226.scala

@@ -0,0 +1,12 @@
+object O1 {
+  opaque type X = Array[X] // error
+}
+
+object O2 {
+  opaque type X = X // error
+}
+
+object O3 { // error
+  opaque type R[X] = T[X]
+  opaque type T[X] = R[X] // error
+}