How to param/return class<T> or null with generics

[darthf1]

Hi!,

I'm trying to achieve something, but I don't know the correct solution. Be it with generics, or just a different implementation.

Playground link: https://phpstan.org/r/f302f0b0-728e-4106-8670-5145a99eaac9

What Im trying to do:

• FirstEvent and SecondEvent classes implement DomainEventInterface.
• $metadata is either NULL, or a DomainEventMetadata.
  • If it is a DomainEventMetadata, I want its array shape to be defined so when I do $metadata->get($key), the $key can be checked by phpstan and the return value becomes known.

Im trying to do this via generics on the interface:

/**
 * @template T of array<string, mixed> | null
 */
interface DomainEventInterface
{
    /**
     * @param (T is null ? null : DomainEventMetadata<T>) $metadata
     */
    public function __construct(
        ?DomainEventMetadata $metadata,
    );

    /**
     * @return (T is null ? null : DomainEventMetadata<T>)
     */
    public function getMetadata(): ?DomainEventMetadata;
}

If T is NULL, then getMetadata() should return NULL, else it should return DomainEventMetadata<T>.

[ondrejmirtes]

Yeah, right now PHPStan does not understand what you're trying to say with this constructor parameter: (T is null ? null : DomainEventMetadata<T>)

There's no PHPDoc tag to instruct PHPStan "infer T as something when doing new Foo()".

What I tried for your example is a "null object" pattern. Basically don't have null in your code but treat "exists" and "does not exist" scenario the same way from the interface PoV: https://phpstan.org/r/640316e7-ab4f-4366-a1ff-734646f15268