mixed template when implementing interface

[marcosh]

Consider the code in https://psalm.dev/r/f3afaafff3

I would expect it to break since $stringCollection is an instance of ArrayCollection<string> which implements Collection<string>. The latter says that the second parameter of add should be a string, while here I can actually pass an int without any reported issues

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/f3afaafff3

<?php

/**
 * @template T
 */
interface Collection
{
    /**
     * @param T $value
     */
    public function add(string $key, $value): self;
}

/**
 * @template T
 * @implements Collection<T>
 */
class ArrayCollection implements Collection
{
    /** @var array<string, mixed> */
    private array $data = [];

    /**
     * @param array<string, mixed> $data
     */
    public function __construct(array $data = [])
    {
        $this->data = $data;
    }

    /**
     * @param mixed $value
     */
    public function add(string $key, $value): Collection
    {
        $newData = array_merge($this->data, [$key => $value]);

        return new self($newData);
    }
}

/** @var ArrayCollection<string> $stringCollection */
$stringCollection = new ArrayCollection();
$stringCollection = $stringCollection->add('one', 1);

Psalm output (using commit 7195275):

INFO: UnusedVariable - 44:1 - Variable $stringCollection is never referenced

[weirdan]

Widening a parameter type (aka parameter contravariance) is totally valid under LSP.

interface I { public function add(string $value): void; }

does not mean that the parameter should be string only. What it means is that any implementors must accept string. They are free to accept any type that is wider than string as well, e.g string|int or mixed.

[marcosh]

right! Thanks for the reply and the clarification!

But what about the case when we have also a return type widening? for example https://psalm.dev/r/32966f69fa

Here I would expect $stringCollection->get to return a string since $stringCollection is an instance of Collection<string>, or am I missing something else?

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/32966f69fa

<?php

/**
 * @template T
 */
interface Collection
{
    /**
     * @param T $value
     */
    public function add(string $key, $value): self;
    
    /**
     * @return T
     */
    public function get(string $key);
}

/**
 * @template T
 * @implements Collection<T>
 */
class ArrayCollection implements Collection
{
    /** @var array<string, mixed> */
    private array $data = [];

    /**
     * @param array<string, mixed> $data
     */
    public function __construct(array $data = [])
    {
        $this->data = $data;
    }

    /**
     * @param mixed $value
     */
    public function add(string $key, $value): Collection
    {
        $newData = array_merge($this->data, [$key => $value]);

        return new self($newData);
    }
    
    /**
     * @return mixed
     */
    public function get(string $key)
    {
        return $this->data[$key];
    }
}

/** @var ArrayCollection<string> $stringCollection */
$stringCollection = new ArrayCollection();
$stringCollection = $stringCollection->add('one', 1);

echo $stringCollection->get('one');

Psalm output (using commit 7195275):

No issues!

[weirdan]

This (https://psalm.dev/r/2bfe7193d0) should be flagged, as T:ArrayCollection as mixed (inherited from the interface) could be narrower than just mixed (declared by the implementor).

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/2bfe7193d0

<?php

/** @template T */
interface Collection
{  
    /** @return T */
    public function get(string $key);
}

/**
 * @template T
 * @implements Collection<T>
 */
abstract class ArrayCollection implements Collection
{  
    /** @return mixed */
    abstract public function get(string $key);
}

Psalm output (using commit 7195275):

No issues!