Fix #8143: add regression test

Author: anatoliykmetyuk

tests/pos/i8143.scala

@@ -0,0 +1,11 @@
+class Reflection(val internal: CompilerInterface) { self => // It works if the self is removed
+  opaque type Flags = internal.Flags
+  object Flags {
+    def EmptyFlags: Flags = internal.Flags_EmptyFlags
+  }
+}
+
+trait CompilerInterface {
+  type Flags
+  def Flags_EmptyFlags: Flags
+}