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.
Merge pull request #3204 from retronym/topic/patmat-classtag-compound

Test case for recently improved unchecked warning