I do not understand why phpstan (at level 9+) flags the concrete factory's makeCollection method (the last class) as returning Collection.

<?php

declare (strict_types=1);

/**
 * @template ElementType
 */
interface CollectionInterface
{
}

/**
 * @template ElementType
 */
interface CollectionFactoryInterface
{
    /**
     * @return CollectionInterface<ElementType>
     */
    public function makeCollection() : CollectionInterface;
}

/**
 * @template ElementType
 * @implements CollectionInterface<ElementType>
 */
class Collection implements CollectionInterface
{
}

/**
 * @template ElementType
 * @implements CollectionFactoryInterface<ElementType>
 */
class CollectionFactory implements CollectionFactoryInterface
{
    /**
     * @return CollectionInterface<ElementType>
     */
    public function makeCollection(): CollectionInterface
    {
        /* phpstan flags this */
        return new Collection();
    }
}

What is the correct way to typehint the code so that both the factory and the return value of the creation method (makeCollection) are compatible with their respective interfaces?

I do not understand why phpstan (at level 9+) flags the concrete factory's makeCollection method (the last class) as returning Collection.

<?php

declare (strict_types=1);

/**
 * @template ElementType
 */
interface CollectionInterface
{
}

/**
 * @template ElementType
 */
interface CollectionFactoryInterface
{
    /**
     * @return CollectionInterface<ElementType>
     */
    public function makeCollection() : CollectionInterface;
}

/**
 * @template ElementType
 * @implements CollectionInterface<ElementType>
 */
class Collection implements CollectionInterface
{
}

/**
 * @template ElementType
 * @implements CollectionFactoryInterface<ElementType>
 */
class CollectionFactory implements CollectionFactoryInterface
{
    /**
     * @return CollectionInterface<ElementType>
     */
    public function makeCollection(): CollectionInterface
    {
        /* phpstan flags this */
        return new Collection();
    }
}

What is the correct way to typehint the code so that both the factory and the return value of the creation method (makeCollection) are compatible with their respective interfaces?

• generics
• factory
• return-type
• phpstan

1 Answer 1

Right now PHPStan reports this error:

Method CollectionFactory::makeCollection() should return CollectionInterface but returns Collection.

Because there’s no guarantee that new Collection is actually CollectionInterfacr<ElementType>. It might actually contain something else.

Four thoughts about this:

1. This is a level 9 error. Level 9 is already pretty strict. You can be on level 8 and still have pretty reliable type checking without these annoyances for you.
2. If the constructor of Collection accepted an array of elements typehinted with array<ElementType>, PHPStan would not complain about this.
3. You can “force” the type by casting with inline PHPDoc @var above the return, making the error to go away.
4. Once this suggestion is implemented, PHPStan will understand the code better and the error will go away: https://github/phpstan/phpstan/issues/6732#issuecomment-1062029088