<?php

class FilterAssert
{
    public static function boolean(callable $filter): void
    {
        switch ($filter) {
            case $filter instanceof Closure:
                $reflection = new ReflectionFunction($filter);
                break;
            case is_object($filter):
                $reflection = new ReflectionMethod($filter, '__invoke');
                break;
            default: 
                throw new InvalidArgumentException(
                    sprintf('Expected Closure or invokable object on callable filter, %s given', gettype($filter))
                );
        }
        
        var_dump($reflection->getReturnType());
    }
}

$filter = new class {
    public function __invoke(int $datum): bool
    {
        return $datum === 1;
    }
};

FilterAssert::boolean($filter);