https://phpstan.org/r/b51ecb93-b683-44b8-85c1-0d53d94bd24f

The PhpStan\dumpType() calls at lines 20 and 23 both produce T0|T1, which makes sense, because the $predicate callable currently returns bool, which doesn't tell PhpStan anything about the type of $value. I'm trying to come up with an @param annotation for $predicate which would:

• allow a callable with a single T0 | T1 argument and returning bool to be passed in
• and treat that callable within the function as though the callable's return value differentiates between T0 and T1 of the provided argument (essentially making the dumpType() call at line 20 return T0 and the one at line 23 return T1)

Perhaps the intent can be better explained by first introducing a non-generic version of this code:
https://phpstan.org/r/efcff1f4-8c95-410c-8907-03ce7171c1aa

This works as expected: the function takes as its argument an iterable containing values of two distinct types, and returns two lists each with values of only one of the input types. This works because the stub definition for the built-in is_string() function includes the @phpstan-assert-if-true annotation, which means that whenever this function is encountered in code, it tells PhpStan something about the type of the passed argument in subsequent code. What I'm trying to come up with is a way to do this generically, so that instead of hard-coding iterable<string | int> and is_string() I can use iterable<T0 | T1> and $predicate().

The next-level version of this would allow me to even do this with more than two "partitions", so that I could have iterable<T0 | T1 | T2 | ...> as the input and array{list<T0>, list<T1>, list<T2>, ...} as the output, with $predicate returning e.g. an int - that would probably need something like @phpstan-assert-if-return 0 T0 $value, @phpstan-assert-if-return 1 T1 $value etc., but that's more of a suggestion for where such a thing could be taken next - I realize this would probably need quite a bit of work in PhpStan. But the original use-case with two types already seems to have almost everything in place - we have @phpstan-assert-if-true and it works with generics - there's just no way to specify it for a @param callable, because the @param annotation is the entire "docblock" for the callable, so there's nowhere to put the @phpstan-assert-if-true annotation.