TypeTest exhaustivity check

[nicolasstucki]

Compiler version

3.0.0-RC2

Minimized code

def test[A, B](a: A|B)(using reflect.TypeTest[Any, A], reflect.TypeTest[Any, B]) =
  a match {  // WARNING: match may not be exhaustive. It would fail on pattern case: _: A, _: B
    case a: A =>
    case b: B =>
  }

Output

match may not be exhaustive. It would fail on pattern case: _: A, _: B

Expectation

Should not warn.

A call to TypeTest.unapply should match all A as an a: A would if A was concrete, therefore it covers the type A. Same for B.

[nicolasstucki]

A possible solution to this is #11186. But as it is a more contentious PR it might be good to special case the fix for this particular feature and generalize it later.