Invalid ImplementedReturnTypeMismatch

[azjezz]

I have encountered a bug in the ImplementedReturnTypeMismatch checker while writing some code today 🙂

the typing here is sane, i have even reimplemented the exact same interfaces in hack lang and was able to type check everything without errors, so this is most likely a psalm bug.

snippet : https://psalm.dev/r/19b73ba81b

hack sample :

namespace MyCollections;

interface ICollection<Tk as arraykey, Tv>
  extends \Countable, \IteratorAggregate<Tv> {
  public function zip<Tu>(Container<Tu> $iterable): ICollection<Tk, (Tv, Tu)>;
}

interface IAccessibleCollection<Tk as arraykey, Tv> extends ICollection<Tk, Tv>
{
  public function zip<Tu>(
    Container<Tu> $iterable
  ): IAccessibleCollection<Tk, (Tv, Tu)>;
}

hack type checker output :

➜  hack-psalm hhvm --version
HipHop VM 4.38.0 (rel)
Compiler: 1577995291_580682453
Repo schema: 9ac3425448b38a0b90b019ba45c2e61eabafa30c
➜  hack-psalm hh_client interfaces.hack
No errors!
➜  hack-psalm 

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/19b73ba81b

<?php
  
/**
 * @template Tk of array-key
 * @template Tv
 *
 * @extends IteratorAggregate<Tk, Tv>
 */
interface Collection extends Countable, IteratorAggregate
{
    /**
     * @psalm-template Tu
     *
     * @psalm-param iterable<Tu> $iterable
     *
     * @psalm-return Collection<Tk, array{0: Tv, 1: Tu}>
     */
    public function zip(iterable $iterable): Collection;
}

/**
 * @template Tk of array-key
 * @template Tv
 *
 * @extends Collection<Tk, Tv>
 */
interface AccessibleCollection extends Collection
{
    /**
     * @psalm-template Tu
     *
     * @psalm-param iterable<Tu> $iterable
     *
     * @psalm-return AccessibleCollection<Tk, array{0: Tv, 1: Tu}>
     */
    public function zip(iterable $iterable): AccessibleCollection;
}

Psalm output (using commit d4ef5c2):

ERROR: ImplementedReturnTypeMismatch - 34:22 - The inherited return type 'Collection<Tk:AccessibleCollection as array-key, array{0: Tv:AccessibleCollection as mixed, 1: Tu:fn-collection::zip as mixed}>' for Collection::zip is different to the implemented return type for AccessibleCollection::zip 'AccessibleCollection<Tk:AccessibleCollection as array-key, array{0: array{0: Tv:AccessibleCollection as mixed, 1: Tu:fn-accessiblecollection::zip as mixed}, 1: Tu:fn-accessiblecollection::zip as mixed}>'

INFO: PossiblyUnusedMethod - 18:21 - Cannot find any calls to method Collection::zip

[muglug]

Can be simplified to

/**
 * @template T1
 */
interface C
{
    /**
     * @psalm-return C<array<int, T1>>
     */
    public function zip(): C;
}

/**
 * @template T2
 * @extends C<T2>
 */
interface AC extends C
{
    /**
     * @psalm-return AC<array<int, T2>>
     */
    public function zip(): AC;
}