array_slice()ing templated array parameter infers array<array-key, mixed>

[someniatko]

https://psalm.dev/r/e5c56d6c48

Versions
without @template: https://psalm.dev/r/e933bf53a9
and without array_slice: https://psalm.dev/r/42bab64fa6

behave as expected

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/e5c56d6c48

<?php

/**
 * @template T as string[]
 * @param T $param
 * @return string[]
 */
function foo(array $param): array
{   
    if (empty($param)) {
        return [];
    }
    
    return array_slice($param, 1);
}

Psalm output (using commit 198fdf8):

INFO: MixedReturnTypeCoercion - 14:12 - The type 'array<array-key, mixed>' is more general than the declared return type 'array<array-key, string>' for foo

INFO: MixedReturnTypeCoercion - 6:12 - The declared return type 'array<array-key, string>' for foo is more specific than the inferred return type 'array<array-key, mixed>'

https://psalm.dev/r/e933bf53a9

<?php

/**
 * @param string[] $param
 * @return string[]
 */
function foo(array $param): array
{   
    if (empty($param)) {
        return [];
    }
    
    return array_slice($param, 1);
}

Psalm output (using commit 198fdf8):

No issues!

https://psalm.dev/r/42bab64fa6

<?php

/**
 * @template T as string[]
 * @param T $param
 * @return string[]
 */
function foo(array $param): array
{   
    if (empty($param)) {
        return [];
    }
    
    return $param;
}

Psalm output (using commit 198fdf8):

No issues!

[weirdan]

That template on the method is a bit useless though, as it does not constrain neither the output nor other parameters.

[someniatko]

@weirdan Yeah, i know, it's just a minimal case.

Here's the production-ish code: https://psalm.dev/r/d16bbba166

p.s. basically reinventing array to linked list conversion here 😄

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d16bbba166

<?php

class Field
{
    public function __construct(public string $value, public ?Field $subField) {}
}

/**
 * @psalm-template T of list<string>
 * @psalm-param T $fieldChunks
 * @psalm-return (T is non-empty-list ? Field : null)
 */
function fieldFromChunks(array $fieldChunks): ?Field
{
    if ($fieldChunks === []) {
        return null;
    }

    return new Field($fieldChunks[0], fieldFromChunks(array_slice($fieldChunks, 1)));
}

$nonNullField = fieldFromChunks([ 'a' ]);
/** @psalm-trace $nonNullField */

Psalm output (using commit 198fdf8):

INFO: PossiblyUndefinedIntArrayOffset - 19:22 - Possibly undefined array offset '0' is risky given expected type 'int'. Consider using isset beforehand.

INFO: MixedArgumentTypeCoercion - 19:55 - Argument 1 of fieldFromChunks expects list<string>, parent type array<array-key, mixed> provided

INFO: Trace - 23:34 - $nonNullField: Field

INFO: UnusedVariable - 22:1 - $nonNullField is never referenced or the value is not used