Property (iterable<T>|null) does not accept iterable<T>

[sanmai]

Bug report

I'm looking at the following errors which read like nonsense:

• Property Example<T>::$input (iterable<T>|null) does not accept iterable<T>.
• Method Example::get() should return iterable<T>|null but returns iterable<T>|null.

Code snippet that reproduces the problem

https://phpstan.org/r/91bc6557-e814-4b06-bc9d-eb9d9ce17342

/**
 * @template T
 */
class Example
{
    /**
     * @var ?iterable<T>
     */
    private $input;
 
    /**
     * @param iterable<T> $input
     */
    public function __construct(iterable $input)
    {
        $this->input = $input;
    }
	
    /** @return ?iterable<T> */
    public function get(): ?iterable
    {
        return $this->input;	
    }
}

Expected output

No errors.

This new issue appeared when upgrading from 0.12.99 to 1.0.2.

[arnaud-lb]

Smaller snippet: https://phpstan.org/r/eafd4309-56cf-4698-b30f-d4cf578f2ad9

Problem disappears when specifying the iterable key to a non-mixed type: https://phpstan.org/r/f2035689-6518-4053-a57a-fe58256d25f9

Seems related to #6418

Probable cause:

• In https://github.com/phpstan/phpstan-src/blob/875f156d386fa789cf543b701d8a719284574fa4/src/Rules/RuleLevelHelper.php#L61 we change the iterable key type form MixedType to StrictMixedType
• In https://github.com/phpstan/phpstan-src/blob/875f156d386fa789cf543b701d8a719284574fa4/src/Type/IterableType.php#L114 we compare an ?iterable<strict-mixed> to array<strict-mixed,mixed>|(Traversable&iterable<strict-mixed,mixed>)
• ArrayType<strict-mixed,mixed>::getIterableKeyType() returns a int|string that is not accepted by strict-mixed

[phpstan-bot]

@sanmai After the latest commit in dev-master, PHPStan now reports different result with your code snippet:

@@ @@
-19: Property Example<T>::$input (iterable<T>|null) does not accept iterable<T>.
-25: Method Example::get() should return iterable<T>|null but returns iterable<T>|null.
+No errors

[phpstan-bot]

@arnaud-lb After the latest commit in dev-master, PHPStan now reports different result with your code snippet:

@@ @@
-9: Function test() should return iterable|null but returns iterable|null.
+No errors

[ondrejmirtes]

Fixed: phpstan/phpstan-src@9f12f0f

[github-actions]

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