Static method return wrongly interpreted, causes return mismatch error

[kevinsmith]

Bug report

Using PHPStan v0.12.25. This may be related to #3273 but seemed different enough that it's worth reporting. If it is the same issue, I figure it might be helpful just to shed light on the bug from another angle.

I'm using a static factory method to create new instances of the generic Collection class. Since it's a static method (and therefore shouldn't be able to inherit the class type variable), I've declared a new method-level type variable and declared that the method returns a Collection of those method-level types. Unfortunately it causes this error:

Method Collection::ofType() should return Collection<U of Immutable> but returns Collection<Immutable>.

Code snippet that reproduces the problem

<?php

interface Immutable {}

/**
 * @template T of Immutable
 */
class Collection
{
    /** @var T[] */
    protected array $values = [];

    /**
     * @param T[] $values
     */
    protected function __construct(array $values = [])
    {
        $this->values = $values;
    }
    
    /**
     * @param class-string<U> $type
     *
     * @return Collection<U>
     *
     * @template U of Immutable
     */
    public static function ofType(string $type) : self
    {
        return new self();
    }
}

https://phpstan.org/r/d388d25d-7ccc-4f4f-89f2-7fafde83fa83

Expected output

I would expect there to be no errors. For reference, this works in Psalm: https://psalm.dev/r/6b3ad129ed

[phpstan-bot]

@kevinsmith After the latest commit in dev-master, PHPStan now reports different result with your code snippet:

@@ @@
-30: Method Collection::ofType() should return Collection<U of Immutable> but returns Collection<Immutable>.
+No errors

[ondrejmirtes]

Fixed: phpstan/phpstan-src@c2b9e71

[github-actions]

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