possibly incorrect MixedMethodCall: "Cannot call method on an unknown class" when using templated class-string param & return types

[SignpostMarv]

This templated scenario seems to trigger an error for two different reasons:

• in the example the class is known, it being class-string<DateTimeImmutable>

• is_a() returns false if either of the first two class-string arguments are unknown, thus the return statement will never be reached: https://3v4l.org/Pd7BJ

• https://getpsalm.org/r/07abe707ba

• using e384fb0

<?php
declare(strict_types=1);

/**
* @template T
*
* @param class-string $maybe
* @param class-string<T> $shouldBe
*
* @return class-string<T>
*
* @throws InvalidArgumentException if $maybe is not an implementation of $shouldBe
*/
function Foo(string $maybe, string $shouldBe) : string {
  if ( ! is_a($maybe, $shouldBe, true)) {
    throw new InvalidArgumentException('nope');
  }
  
  return $maybe;
}

Foo(DateTimeImmutable::class, DateTimeImmutable::class)::getFromMutable(new DateTime());

[weirdan]

I think it could be simplified further (https://getpsalm.org/r/94b619a6ac):

<?php
/**
* @template T
*
* @param class-string<T> $shouldBe
* @return class-string<T>
*/
function identity(string $shouldBe) : string {  return $shouldBe; }

identity(DateTimeImmutable::class)::getFromMutable(new DateTime());

[SignpostMarv]

@weirdan does psalm internals make a distinction between class-string for a known class and class-string for an unknown class ?

[weirdan]

Depends on what you mean by that, I suppose. class-string<Exception> and just class-string do result in different behavior in some places.

However in this case the return type is inferred to be class-string<> (https://getpsalm.org/r/ecaf471209), which is certainly not something I'd expect to see.

[SignpostMarv]

@weirdan I was using class-string<void> for known unknown class testing, but it's detected as a reserved word now :p

i.e. void is a reserved word thus can never be a class.

[muglug]

class-string is equivalent to class-string<object>