DateInterval::days should be refined to not have false values when returned from DateTimeInterface::diff

[stof]

Bug report

https://www.php.net/manual/en/class.dateinterval.php#dateinterval.props.days documents that a DateInterval returned by DateTimeInterface::diff always have days set (as it can compute it in this case).

It would be great if phpstan could automatically refine the value in this case, to avoid having to add explicit assert($interval->days !== false) just after.

As DateInterval is a mutable object in PHP, this should be implemented using a refined type (that would be invalidated when passing that object to an impure function or when assigning a value to the property) rather than by changing the property type (i.e. exactly what happens when writing this assert explicitly).

Code snippet that reproduces the problem

https://phpstan.org/r/0e888080-29db-4ff8-8f03-58a7e726c2b1

Expected output

No error being reported.

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No response

[VincentLanglet]

NB: Kinda similar to #1452

[stof]

#1452 is about having a more precise type for DateInterval::format(), which is probably harder to implement (and might actually benefit from the refined type of DateInterval::$days to refine the type of ->format('%a') in a return type extension).

[VincentLanglet]

I didn't said it was a duplicate, but to me it's related because both are linked to the fact the DateInterval is coming from diff and not from a new DateInterval call.
As soon as we're able to "flag" the DateInterval, we'll be able to solve both issue.

The format dynamicReturnType already exists.

[phpstan-bot]

Closed via merging phpstan/phpstan-src#5588