Make DateTimeInterface::format return string|false

[RikudouSage]

While the official signature (public DateTimeInterface::format ( string $format ) : string) doesn't contain the false part, in return value description you can read:

Returns the formatted date string on success or FALSE on failure.

[ondrejmirtes]

When the failure can occur? I want to avoid that everyone would have to check that they don't have false before manipulating the string.

[RikudouSage]

Don't know what's the condition for it to actually return false, but the problem is when extending some of the classes - if you return anything else than string phpstan tells you it should return a string (in my case I was returning string|false which is valid as per documentation).

[ondrejmirtes]

Please report that as a bug with phpstan.org link. Thanks.