public function type(string $classType)
    {
        $baseType = $this->class ? $this->class->getClassName() : TypedObject::class;
        if (!is_a($classType, $baseType, true)) {
            throw InvalidArgumentException::format(
                'Invalid call to %s: class type must be an subclass of %s, %s given',
                __METHOD__, $baseType, $classType
            );
        }

        /** @var TypedObject $classType */
        $this->class = $classType::definition();
    }