Add regression test

Author: EugeneFlesselle

tests/neg/i19949.scala

@@ -0,0 +1,9 @@
+
+trait T[N]:
+  type M = N match
+    case 0 => Any
+
+val t: T[Double] = new T[Double] {}
+val x: t.M = "hello" // error
+
+val z: T[Double]#M = "hello" // error