Allow @phpstan-assert to narrow to a truthy or falsy value

[BHSPitMonkey]

Feature request

Hi! I have a project with some commonly-used assertion functions that will throw if the given value converts to true (or false). Taking a cue from the examples on Narrowing Types > Custom type-checking functions and methods, I'd love to be able to do something like this:

/** @phpstan-assert truthy $value */
public function throwIfFalsy(mixed $value): void
{
    if (!$value) {
        throw new InvalidTypeException();
    }
}

This would allow, for example, narrowing a variable from SomeClass|null to SomeClass after this function was called. (Or, conversely, a function set up in the opposite way could narrow SomeClass|null to null.)

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[ondrejmirtes]

Thank you for your kind words!

You can already do this today: @phpstan-assert !0|0.0|''|'0'|array{}|false|null $value

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