Incorrect Pattern Match Exhaustivity Warning #9354
Description
Minimized code
See cats at this position:
https://github.com/typelevel/cats/blob/5ac10adbc1714e025ae2b6e9429028d5aea4d309/core/src/main/scala/cats/data/AndThen.scala#L95
private def runLoop(start: T): R = {
var self: AndThen[Any, Any] = this.asInstanceOf[AndThen[Any, Any]]
var current: Any = start.asInstanceOf[Any]
var continue = true
while (continue) {
self match {
case Single(f, _) =>
current = f(current)
continue = false
case Concat(Single(f, _), right) =>
current = f(current)
self = right.asInstanceOf[AndThen[Any, Any]]
case Concat(left @ Concat(_, _), right) =>
self = left.rotateAccum(right)
}
}
current.asInstanceOf[R]
}This gives a warning that AndThen.Single(_, _) and AndThen.Concat(AndThen.Single(_, _), _) would fail, but those are the first and second elements of the match respectively:
[warn] -- [E029] Pattern Match Exhaustivity Warning: /home/oscarboykin/oss/cats/core/src/main/scala/cats/data/AndThen.scala:95:6
[warn] 95 | self match {
[warn] | ^^^^
[warn] |match may not be exhaustive.
[warn] |
[warn] |It would fail on pattern case: AndThen.Single(_, _), AndThen.Concat(AndThen.Single(_, _), _)
I guess this may have to do with the specific type parameters changing in this loop. Note we cast to AndThen[Any, Any] but the internal types will have different types.
If that is the problem, maybe we could have a clearer error message because it is confusing to see a claim that a pattern wouldn't match that is explicitly in the match.
cc @travisbrown who has spent time on dotty and cats.