Missing key not reported after generic function

[iliubinskii]

Bug report

Missing key not reported after generic function, but reported for directly defined variable.

See it in playground:
https://phpstan.org/r/bf679482-96aa-49b2-8ac9-6f77a3a24fb1

Code snippet that reproduces the problem

<?php 

declare(strict_types = 1);

/**
 * @template T of int|string
 *
 * @param array<T,mixed> $arr
 *
 * @return array<T,string>
 */
function strings(array $arr): array
{
  // @phpstan-ignore-next-line
  return $arr;
}

$x = ['a' => 1];

$y = strings($x);

var_dump($x['b']);
var_dump($y['b']);

Expected output

22 | Offset 'b' does not exist on array{a: 1}.
23 | Offset 'b' does not exist on array{a: string}.

Did PHPStan help you today? Did it make you happy in any way?

Yes, great tool.

[TemirkhanN]

I don't think that it ever intended to work like that. Generics assert types - not values.

Would you expect the following declaration to check
a. return instanceof $arg
b. return === $arg

?

/**
* @param InputType $arg
* 
* @return InputType
function cloneIt(object $arg): object {
      return clone $arg;
}

[iliubinskii]

I don't think that it ever intended to work like that. Generics assert types - not values.

$x = ['a' => 1];
var_dump($x['b']);

PHPStan reports this.
So, it understands that $x's type is array<'a',int>, and that key type is 'a' (this is enum type).

$y = strings($x);

Here, $x's type is <'a',int>.

@param array<T,mixed> $arr

In this param definition, T is key type.
Key type of $x is 'a' (enum).
So, we assume that T = 'a' (enum)

@return array<T,string>

By substituting T, we get return type array<'a',string> (array with enum key).

var_dump($y['b']);

Should be reported because $y type is <'a',string>

---

I understand that implementing this may be a lot of work.
But it may also be a small fix.
I can't know this.
So I am just showing what I would expect after working with typescript.

[ondrejmirtes]

Last night I came up with the idea that we mostly shouldn't generalize the generic type variables, except when they're in object generics, like Foo<int>.

Here's the resulting PR: phpstan/phpstan-src#2818

We can't do this for objects, because I want new Collection([1, 2, 3]) to become Collection<int>. This could be improved in the future thanks to this suggestion: #6732 (comment) (but it's pretty complex to implement so for now it has to wait).

The new behaviour now only applies to bleeding edge (https://phpstan.org/blog/what-is-bleeding-edge) so definitely enable it to get the taste of the future 👍

[github-actions]

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