$next = function (string $input) : int {
    return strlen($input); // simplistic example, ignore the fact that it is redundant
};

$a = new A($next, 'foo');

$a(); // int - can be inferred, in theory

/**
 * @psalm-template XType
 * @psalm-template ResultType
 */
final class A extends Combinator
{
    /**
     * @psalm-param callable(XType) : ResultType $f
     * @psalm-param XType $x
     */
    public function __construct(callable $f, $x)
    {
        // ...
    }

    /** @psalm-return ResultType */
    public function __invoke()
    {
        // ...
    }

    /** @psalm-return callable() : ResultType */
    public static function closure(): callable
    {
        return static function (callable $f, $x) {
            return (new self($f, $x))();
        };
    }
}