What does the __completion_signature concept in stdexec (senders/ receivers) do?

Question

I'm looking at the reference implementation of P2300 (the senders and receivers proposal).

I'm having trouble understanding some of the code:

namespace __compl_sigs {
    template <same_as<set_value_t> _Tag, class _Ty = __q<__types>, class... _Args>
    __types<__minvoke<_Ty, _Args...>> __test(_Tag (*)(_Args...));

    template <same_as<set_error_t> _Tag, class _Ty = __q<__types>, class _Error>
    __types<__minvoke<_Ty, _Error>> __test(_Tag (*)(_Error));

    template <same_as<set_stopped_t> _Tag, class _Ty = __q<__types>>
    __types<__minvoke<_Ty>> __test(_Tag (*)());

    template <class, class = void>
    __types<> __test(...);

    template <class _Tag, class _Ty = void, class... _Args>
    void __test(_Tag (*)(_Args...) noexcept) = delete;

    template <class _Sig>
    concept __completion_signature = __typename<decltype(__compl_sigs::__test((_Sig*) nullptr))>;
} // namespace __compl_sigs

using __compl_sigs::__completion_signature;

// [...]

template <__compl_sigs::__completion_signature... _Sigs>
struct completion_signatures {
    // Uncomment this to see where completion_signatures is
    // erroneously getting instantiated:
    //static_assert(sizeof...(_Sigs) == -1u);
};

Fundamentally, can someone give me an example for a sender and what __types<__minvoke<_Ty, _Args...>>, __types<__minvoke<_Ty, _Error>> and __types<__minvoke<_Ty>> actually mean? I understand that they relate to set_value, set_error and set_stopped.

Finally, what is the use for completion_signatures?