mb_str_split result can be an empty array

[mvorisek]

Bug report

Code snippet that reproduces the problem

demo: https://phpstan.org/r/b78dcbd0-6791-4090-b50b-599fa5593ac2

Expected output

no error

[herndlm]

Maybe your snippet is not a good example, but I think in this case PHPStan actually knows that the array is always empty if you split '' and the error is correct, right?

[mvorisek]

You are right, I updated the example. The issue is present when non-empty-string type is passed.

[herndlm]

Ok, that looks better, I guess the ''|'x' is not handled correctly via mb_str_split in https://github.com/phpstan/phpstan-src/blob/1.8.0/src/Type/Php/StrSplitFunctionReturnTypeExtension.php. Maybe L89 and the stuff below should somehow use TypeTraverser::map instead or something like that, but I did not take a close look. Do you want to @mvorisek? :)

[herndlm]

I continued looking at this and apparently there are 2 issues here.
The first one being that the extension can't handle compound types like Union basically. I have a fix for that ready.
I don't fully understand the second one though. Apparently mb_str_split can return an empty array while str_split won't. Any ideas why? E.g. https://3v4l.org/r6sMe

Maybe I'll open a PR already for the first thing while figuring the second out hmm. I just could adapt all default types for mb_str_split, but I'd like to understand it first.

[mvorisek]

Apparently mb_str_split can return an empty array while str_split won't. Any ideas why? E.g. https://3v4l.org/r6sMe

This is how PHP works since the mb_str_split function has been introduced in PHP 7.4. Yes, it is confusing, and I opened an issue in php-src for it. But for now, mb_str_split can return an empty array for '' while str_split returns at least one element.

[herndlm]

Ok thx, I saw in an old PR the info that this was changed in psalm as well to a non-empty-array but now I found vimeo/psalm#6493. That's enough for me to revert it back here too :)

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