Forbid iterating over generators with non-nullable send()

[weirdan]

Fixes #6985

[weirdan]

Some failures look valid, including the test-with-real-projects/PSL, as declaring TSend as void seems wrong. TSend is used as both the send() parameter type and the yield return type, and void as a parameter type is rather misleading. Also, as described in the linked issue, foreach() invokes send(null) during iteration, so a generator with TSend=void is rather useless - it cannot be iterated over.

/cc: @azjezz

[weirdan]

The only remaining failures are for Generator<..., ..., void, ...> - we have to decide whether we want to allow that (and treat it as null) or reject it (see above).

[orklah]

I may be wrong on that, but I believe Psalter itself used to (or still) generate signature with void generators like that

Anyway, maybe we could mimic the behavior of phpstan for this?

I don't see the need of creating one more difference for such a niche case.

[weirdan]

Anyway, maybe we could mimic the behavior of phpstan for this?

PHPStan doesn't flag iteration over Generator<..., ..., void, ...>, but not because it special cases void, but because it's not aware that foreach invokes send(null): https://phpstan.org/r/86c02395-e906-4493-a354-99a39d135323, https://3v4l.org/ONh6K

Just like Psalm before this pull request.

@ondrejmirtes WDYT?

My preferred approach, I think, would be to allow people to shoot themselves in the foot specify Generator<..., ..., void, ...> for the time being but then flag all iterations over this generator. Then in a couple of years start flagging Generator<..., ..., void, ...> just like we flag @param void now.

[weirdan]

I'm generally satisfied with how it works now.

@veewee heads up, this will break PSL because Awaitable::iterate() produces a generator that nominally cannot be iterated over:
https://github.com/azjezz/psl/blob/b85e5e1c8e2385bf108bb3a09fb36b459dbc1a07/src/Psl/Async/Awaitable.php#L52-L54

[veewee]

Thanks for the heads-up. It should be fixed in azjezz/psl#444