Why is there no forward declaration in concepts c++?

When I try this example:

template <typename T>
concept only_int = std::same_as<T, int>;

int add_ints(only_int auto&&... args) {
    return (std::forward<decltype(args)>(args) + ... + 0);
}

It works... but when I only declare it like this:

template <typename T>
concept only_int;

...

// defined later on...

It would throw compilation errors.

Is this a missing feature? or it is intended to leave like this?