Fix #8932: add test

Author: liufengyun

tests/neg-custom-args/isInstanceOf/i8932.scala

@@ -0,0 +1,11 @@
+sealed trait Foo[+A]
+case class Bar[A]() extends Foo[A]
+
+class Dummy extends Bar[Nothing] with Foo[String]
+
+def bugReport[A](foo: Foo[A]): Foo[A] =
+  foo match {
+    case bar: Bar[A] => bar    // error
+  }
+
+def test = bugReport(new Dummy: Foo[String])