Invalid return type for template where param and return are the same array

[M1ke]

https://psalm.dev/r/c75c90c4ec

The issue here is that we're wanting the docblock to declare that the function must return the input param. Psalm instead requires us to permit the function to return either the input value, or an alternative array that shares a similar type description.

This means that the common goal of "a function that takes an array, modifies it and returns it" can't be represented without also permitting the function to take an array, discard it and return something that looks like it.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/c75c90c4ec

<?php

/**
 * @template T of array<int, string>
 * @psalm-param T $a
 * @psalm-return T
 */
function takesAnInt(array $a) {
    for ($i=random_int(0,10); $i<10; $i++){
        $a[$i] = md5((string) $i);
    }
    
    return $a;
}

Psalm output (using commit ee1c36c):

ERROR: InvalidReturnStatement - 13:12 - The inferred type '(T:fn-takesanint as array<int, string>)|non-empty-array<int, string>' does not match the declared return type 'T:fn-takesanint as array<int, string>' for takesAnInt

ERROR: InvalidReturnType - 6:18 - The declared return type 'T:fn-takesanint as array<int, string>' for takesAnInt is incorrect, got '(T:fn-takesanint as array<int, string>)|non-empty-array<int, string>'

[M1ke]

If we unset the first key instead then Psalm has no issues with modifying it

https://psalm.dev/r/d6609968af

However if we modify an internal item (even if we explicitly check it exists first) the same issue is thrown.

https://psalm.dev/r/c75c90c4ec

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d6609968af

<?php

/**
 * @template T of array<int, string>
 * @psalm-param T $a
 * @psalm-return T
 */
function takesAnInt(array $a) {
    unset($a[random_int(0,10)]);
    
    return $a;
}

Psalm output (using commit ee1c36c):

No issues!

https://psalm.dev/r/c75c90c4ec

<?php

/**
 * @template T of array<int, string>
 * @psalm-param T $a
 * @psalm-return T
 */
function takesAnInt(array $a) {
    for ($i=random_int(0,10); $i<10; $i++){
        $a[$i] = md5((string) $i);
    }
    
    return $a;
}

Psalm output (using commit ee1c36c):

ERROR: InvalidReturnStatement - 13:12 - The inferred type '(T:fn-takesanint as array<int, string>)|non-empty-array<int, string>' does not match the declared return type 'T:fn-takesanint as array<int, string>' for takesAnInt

ERROR: InvalidReturnType - 6:18 - The declared return type 'T:fn-takesanint as array<int, string>' for takesAnInt is incorrect, got '(T:fn-takesanint as array<int, string>)|non-empty-array<int, string>'

[kkmuffme]

class-string-map https://psalm.dev/docs/annotating_code/type_syntax/utility_types/#class-string-mapt-of-foo-t does exactly this for class instances.
We would need a more generic type to be used for cases of "normal" arrays (and lists)
To get this to work correctly too https://psalm.dev/r/c39fbb3124

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/c39fbb3124

<?php

/**
 * @template T of int
 * @psalm-param array<T, string> $a
 * @psalm-return array<T, string>
 */
function takesAnInt($a) {
    foreach ( $a as $key => $value ) {
     	$a[ $key ] = 'hello' . $value;   
    }
    
    return $a;
}

$x = takesAnInt( array( 10 => 'bar', 15 => 'world' ) );
/** @psalm-trace $x */;

Psalm output (using commit 086b943):

INFO: Trace - 17:23 - $x: array<10|15, string>

INFO: UnusedVariable - 16:1 - $x is never referenced or the value is not used