Fix #1828: add test

Author: liufengyun

tests/neg-custom-args/isInstanceOf/1828.scala

@@ -0,0 +1,9 @@
+class Test {
+  def remove[S](a: S | Int, f: Int => S):S = a match {
+    case a: S => a             // error
+    case a: Int => f(a)
+  }
+
+  val t: Int | String = 5
+  val t1 = remove[String](t, _.toString)
+}