Test case for recently improved unchecked warning

Prior to https://github.com/scala/scala/pull/2848, the enclosed
test compiled without warning and printed:

    true
    false

Features interacting:
  - implicit class tags to enable type patterns on abstract types
  - type tests on compound types.

I think the unchecked warning is acceptable for now.

1 files changed, 17 insertions, 0 deletions

diff --git a/test/files/neg/patmat-classtag-compound.scala b/test/files/neg/patmat-classtag-compound.scala
new file mode 100644
index 0000000000..e2d0df0a02
--- /dev/null
+++ b/test/files/neg/patmat-classtag-compound.scala
@@ -0,0 +1,17 @@
+object Test extends App{
+  trait Bar
+  trait Foo
+  // Failed to give an unchecked warning pre: https://github.com/scala/scala/pull/2848
+  //
+  // Features interacting:
+  //   - implicit class tags to enable type patterns on abstract types
+  //   - type tests on compound types.
+  //
+  // We could try make these work together, but an unchecked warning is okay for now.
+  def x[A: reflect.ClassTag](a: Any): Boolean = a match{
+    case b: A with Bar => true
+    case _ => false
+  }
+  println(x[Foo](new Bar{}))
+  println(x[String](""))
+}