Bug report

I'm attempting to implement a class that has caching for the method's return values.

For example, given a class Cat that has three methods:

• count_descendants(): int
• is_hungry(): bool
• get_name(): string

I have a private $result_cache class member variable which is typed as follows:

 * @phpstan-type ResultCache array{
 *                               count_descendants?: int<1, max>,
 *                               is_hungry?: bool,
 *                               name?: non-empty-string
 *                           }

There is a get_cached_result method which is used as follows:

	public function count_descendants(): int {
		return $this->get_cached_result( __FUNCTION__, static function () {
			return 123; // TODO: Some expensive operation.
		} );
	}

And finally the get_cached_result method itself is implemented as follows:

	/**
	 * Get cached result.
	 *
	 * @template K of key-of<ResultCache>
	 * @phpstan-param K                         $method   Method.
	 * @phpstan-param Closure(): ResultCache[K] $callback Callback.
	 * @phpstan-return ResultCache[K]
	 *
	 * @param string  $method   Method name.
	 * @param Closure $callback Callback to get the result.
	 * @return mixed
	 */
	private function get_cached_result( string $method, Closure $callback ) {
		if ( array_key_exists( $method, $this->result_cache ) ) {
			return $this->result_cache[ $method ];
		}
		$result = $callback();
		$this->result_cache[ $method ] = $result;
		return $result;
	}

The problem is on this line:

$this->result_cache[ $method ] = $result;

PHPStan is flagging this as an error saying that $result is:

array{
    count_descendants?: bool|int<1, max>|non-empty-string,
    is_hungry?:         bool|int<1, max>|non-empty-string, 
    name?:              bool|int<1, max>|non-empty-string
}

Which does not match the type of $result_cache which is (as shown above):

array{
    count_descendants?: int<1, max>,
    is_hungry?:         bool,
    name?:              non-empty-string
}

What seems to be happening is PHPStan thinks the type of $result is a union of all possible types for the array values in the ResultCache type. But this seems like it should not be because I specified that the supplied closure returns the type ResultCache[K] where K is one of key-of<ResultCache>.

Is this a bug in PHPStan? Or am I just doing generics wrong?

Code snippet that reproduces the problem

https://phpstan.org/r/b203ec8c-2d70-469c-b24f-9e7143cff72d

Expected output

I believe this is a false positive. No error should have been detected.

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