Merge pull request #2561 from dotty-staging/fix-#1650

Fix #1650: Add regression test

Author: DarkDimius

tests/neg/i1650.scala

@@ -0,0 +1,5 @@
+object Test {
+  test4(test4$default$1) // error
+  def test4[T[P]](x: T[T[List[T[X forSome { type X }]]]]) = ??? // error // error
+  def test4$default$1[T[P]]: T[Int] = ???
+}