Fix #8711: add test

Author: liufengyun

tests/neg-custom-args/fatal-warnings/i8711.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-custom-args/fatal-warnings/i8711.scala:7:9 ---------------------------------------------------------
+7 |    case x: B => x // error: this case is unreachable since class A is not a subclass of class B
+  |         ^
+  |         this case is unreachable since type A and class B are unrelated
+-- Error: tests/neg-custom-args/fatal-warnings/i8711.scala:12:9 --------------------------------------------------------
+12 |    case x: C => x // error
+   |         ^
+   |         this case is unreachable since type A | B and class C are unrelated

tests/neg-custom-args/fatal-warnings/i8711.scala

@@ -0,0 +1,15 @@
+class A
+class B
+class C
+
+object Test {
+  def foo(x: A) = x match {
+    case x: B => x // error: this case is unreachable since class A is not a subclass of class B
+    case _ =>
+  }
+
+  def bar(x: A | B) = x match {
+    case x: C => x // error
+    case _ =>
+  }
+}