Cannot combine generic key types when @template is limited to array-key

[Shira-3749]

Bug report

When I define a generic key type as @template TKey of array-key, PHPStan doesn't seem to want to combine it with other types via @return:

/**
 * @template TOtherKey of array-key
 * @template TOtherValue
 *
 * @param iterable<TOtherKey, TOtherValue> ...$iterables
 * @return self<TKey|TOtherKey, TValue|TOtherValue>
 */
function merge(iterable ...$iterables): self;

Calling this method does not combine TKey with TOtherKey - it just keeps it as TKey:
https://phpstan.org/r/31d3d483-7e0c-4239-8c88-67cf00c807ae

This seems to work fine in Psalm: https://psalm.dev/r/8671dd080b

Using string|int instead of array-key seems to "fix" this:
https://phpstan.org/r/edf51290-fe27-4024-b07d-3e7c327e5825

And so does removing the type entirely:
https://phpstan.org/r/ff4f41fa-691c-4fe4-8837-ef0ec2179c04

Code snippet that reproduces the problem

https://phpstan.org/r/31d3d483-7e0c-4239-8c88-67cf00c807ae

Expected output

-Dumped type: Map<string, bool|int>
+Dumped type: Map<int|string, bool|int>
-Dumped type: Map<string, bool|int|stdClass>
+Dumped type: Map<int|string, bool|int|stdClass>

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