Address review

Author: liufengyun

tests/neg-custom-args/isInstanceOf/i5826b.scala

@@ -0,0 +1,11 @@
+class Foo {
+  def test1[A]: List[Int] | A => Int = {
+    case ls: List[_] => ls.head       // error
+    case _ => 0
+  }
+
+  def test2[A]: List[Int] | A => Int = {
+    case ls: List[_] => ls.size
+    case _ => 0
+  }
+}