Psalm complains when passing an array with extra properties

[BenMorel]

Repro: https://psalm.dev/r/440c1cd447

/**
 * @psalm-template K
 * @psalm-template V
 */
class Collection {}

/**
 * @psalm-param Collection<int, array{code: string}> $items
 */
function test(Collection $items): void {}

/** @var Collection<int, array{code: string, value: string}> $value */
test($value);

Result:

ERROR: InvalidArgument - 17:6 - Argument 1 of test expects Collection<int, array{code: string}>, Collection<int, array{code: string, value: string}> provided

If I change Collection for an array, it works fine: https://psalm.dev/r/0ae6a8d4a3

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/440c1cd447

<?php

/**
 * @psalm-template K
 * @psalm-template V
 */
class Collection {}

/**
 * @psalm-suppress UnusedParam
 * @psalm-param Collection<int, array{code: string}> $items
 */
function test(Collection $items): void {}

/** @var Collection<int, array{code: string, value: string}> $value */

test($value);

Psalm output (using commit 3817193):

ERROR: InvalidArgument - 17:6 - Argument 1 of test expects Collection<int, array{code: string}>, Collection<int, array{code: string, value: string}> provided

https://psalm.dev/r/0ae6a8d4a3

<?php

/**
 * @psalm-suppress UnusedParam
 * @psalm-param array<int, array{code: string}> $items
 */
function test(array $items): void {}

/** @var array<int, array{code: string, value: string}> $value */

test($value);

Psalm output (using commit 3817193):

No issues!

[weirdan]

Generics are invariant, so you cannot pass Collection of a subtype where Collection of a supertype is expected. You would have to mark corresponding type parameter covariant for that to work: https://psalm.dev/r/015291e4ca

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/015291e4ca

<?php

/**
 * @psalm-template K
 * @psalm-template-covariant V
 */
class Collection {}

/**
 * @psalm-suppress UnusedParam
 * @psalm-param Collection<int, array{code: string}> $items
 */
function test(Collection $items): void {}

/** @var Collection<int, array{code: string, value: string}> $value */

test($value);

Psalm output (using commit 90fd1c5):

No issues!

[BenMorel]

Thanks for the pointer! I don't understand why generics are not covariant by default, though: what's a use case when covariance would not be desirable?

[weirdan]

Because of this: #1603

[weirdan]

The same example is also discussed in the docs: https://psalm.dev/docs/annotating_code/templated_annotations/#template-covariance

[BenMorel]

Thank you, I understand the issue now. But can't the covariance / contravariance be inferred from the use case though: when it's used as a parameter or as a return type?

Why does it works with plain arrays?

[weirdan]

Array that you receive as a parameter is copied once you change it. It does not affect the array that was passed.