Fix #5278: Add regression test

Author: nicolasstucki

tests/neg/i5278.scala

@@ -0,0 +1,11 @@
+trait T { self: {type M = Int} =>
+    type T = self.M
+    def lift(x: Int): T = x
+  }
+
+  class Test {
+    val t = new T {
+      type M = Int
+    }
+    t.lift(1): t.T // error
+  }