Wrong type inference

[marcosh]

Consider this snippet of code (see https://psalm.dev/r/422d9413c9)

/**
 * @template S
 * @template T
 * @template A
 * @template B
 */
final class Prism
{
	/** @var callable(B): T */
    private $review;

    /** @var callable(S): Either<T, A> */
    private $preview;

    /**
     * @template C
     * @template D
     * @param Prism<A, B, C, D> $that
     */
    public function compose(Prism $that): void
    {
        /** @psalm-trace $preview */
        $preview = $that->preview;
    }
}

Psalm says that the type of $preview is callable(C): Either<D, C>, while I'm expecting it to be callable(A): Either<B, C>.

Am I doing something wrong or is Psalm inferring the wrong type here?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/422d9413c9

<?php

/**
 * @template E
 * @template F
 */
final class Either
{}

/**
 * @template S
 * @template T
 * @template A
 * @template B
 */
final class Prism
{
	/** @var callable(B): T */
    private $review;

    /** @var callable(S): Either<T, A> */
    private $preview;

    /**
     * @template C
     * @template D
     * @param Prism<A, B, C, D> $that
     */
    public function compose(Prism $that): void
    {
        /** @psalm-trace $preview */
        $preview = $that->preview;
    }
}

Psalm output (using commit 724b25b):

INFO: Trace - 32:9 - $preview: callable(C:fn-prism::compose as mixed):Either<D:fn-prism::compose as mixed, C:fn-prism::compose as mixed>

INFO: UnusedVariable - 32:9 - Variable $preview is never referenced

ERROR: MissingConstructor - 19:13 - Prism has an uninitialized property Prism::$review, but no constructor

ERROR: MissingConstructor - 22:13 - Prism has an uninitialized property Prism::$preview, but no constructor

[muglug]

Reduced to https://psalm.dev/r/565bb30e0b

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/565bb30e0b

<?php

/**
 * @template A
 * @template B
 */
final class Foo
{
	/** @var Traversable<A, B> */
    public $traversable;
    
    /**
     * @param Foo<B, A> $flipped
     * @return Traversable<B, A>
     */
    public function getFlippedTraversable(Foo $flipped): Traversable
    {
        return $flipped->traversable;
    }
}

Psalm output (using commit b2382cd):

ERROR: InvalidReturnStatement - 18:16 - The inferred type 'Traversable<A:Foo as mixed, B:Foo as mixed>' does not match the declared return type 'Traversable<B:Foo as mixed, A:Foo as mixed>' for Foo::getFlippedTraversable

ERROR: InvalidReturnType - 14:16 - The declared return type 'Traversable<B:Foo as mixed, A:Foo as mixed>' for Foo::getFlippedTraversable is incorrect, got 'Traversable<A:Foo as mixed, B:Foo as mixed>'

ERROR: MissingConstructor - 10:12 - Foo has an uninitialized property Foo::$traversable, but no constructor

[muglug]

Ah, I just fixed it for methods

[marcosh]

that was fast, thank you!