Conditional return type including template type resolved in wrong place

[jankal]

Bug report

I am returning a class-string from a method which could also return null in the default generic case (using a conditional return type declaration). The returned class-string type is not checked in the context of the child class but in the generic context of the parent/definer and thus not part of the generic template variable.

Code snippet that reproduces the problem

https://phpstan.org/r/ea335711-73a7-4d4e-9647-9212eef29f75

Expected output

This code shouldn't have any issues as DeleteActionType implements DataClassAwareFormTypeInterface<DeleteActionParameters> and DeleteActionParameters is of Struct

Did PHPStan help you today? Did it make you happy in any way?

Most of the time, PHPStan is more intelligent than I am - discovering issues with covariance and contravariance I otherwise would've missed :)

[jankal]

I would rather have used some kind of optional generic template syntax... like this:

/**
* @template ?TParam of Struct
*/
interface EmailActionInterface
{
    /** @return (TParams is null ? null : class-string<DataClassAwareFormTypeInterface<TParams>>) */
    public static function getParameterTypeClass(): ?string;

    /**
     * @param (TParams ?? null) $params
     * @return void
     */
    public function __invoke(?Struct $params = null): void;
}

But that syntax apparently does not work.

[staabm]

Not 100% sure, but I think you could use template defaults: phpstan/phpstan-src#3457

[jankal]

@staabm yeah, that works for setting the default in the generic and omitting the generic parameter when the default should be used.

The actual bug is about PHPStan apparently checking the return type in the wrong context (error says, the return type of that function is not part of the template parameter... which it is as it is of Struct).

[jankal]

It just occurred to me: when I add a @return class-string<...> annotation to the method, PHPStorm doesn't complain anymore...

https://phpstan.org/r/e47e606f-c444-45f6-aada-edf73d587f19

Looks like a bug to me.