Unexpected list of failing pattern cases in non-exhaustivity-warning

[ansvonwa]

Compiler version

3.0.0, 3.1.0, 3.1.1-RC1, 3.1.2-RC1-bin-20211102-82172ed-NIGHTLY

Minimized code

Vector() match {
  case v @ Seq() => println(v.getClass)
}

(Same as in #13931 without the case _ =>. Related problems?)

Output

[warn] 3 |    Vector() match {
[warn]   |    ^^^^^^^^
[warn]   |match may not be exhaustive.
[warn]   |
[warn]   |It would fail on pattern case: Vector0, _: Vector2[Nothing], _: Vector3[Nothing], _: Vector4[Nothing], _: Vector5[Nothing], _: Vector6[Nothing]
[warn]   |(More unmatched cases are elided)
[warn] one warning found
[warn] one warning found
[info] running run 
class scala.collection.immutable.Vector0$

Expectation

Vector0 not being in the list of "failing" patterns, as the empty Vector() is matched.

Instead, _: Vector1[Nothing] should be there?

[ansvonwa]

In Scala 2.13.7, Vector1() is contained in the list as expected, but also Vector0:

[warn] .../src/main/scala/Foo.scala:3:11: match may not be exhaustive.
[warn] It would fail on the following inputs: Vector0, Vector1(), Vector2(), Vector3(), Vector4(), Vector5(), Vector6()
[warn]     Vector() match {
[warn]           ^
[warn] one warning found
[info] running Foo 
class scala.collection.immutable.Vector0$

[KacperFKorban]

Can be related to #13931

[dwijnand]

There's no knowledge in the types or the trees that could inform the compiler that Vector0 is covered by the use of Seq(). And Vector1 is missing because it's not in the first subclasses, and the exhaustivity checker only ever shows 6 counter examples in Scala 3.

So this is working as expected, but you could argue and someone might want to improve the status quo, on the following things:

1. When exhaustivity fails due to arity (and only arity, I think is necessary) then present counter examples based on arity, i.e. It would fail on pattern case: Seq(_), Seq(_, _).
2. Remove the cut off on counter examples?
3. Make subclasses (and thus counter-exampes) iterate in declaration or name order, rather than Vector1 looking like it's intentionally not mentioned.
4. Have a way to define the arity of subclasses, such that the exhaustivity checker could use that information (a language improvement).