Explain repeat_until completion_signatures

[BartolomeyKant]

I'm trying to use repeat_until and assign final sender to any_sender. So I need to specify sender's completion_signatures properly. But using repeat_until generates a mess in completion_signature.

I made a short example of that I want to achieve. https://godbolt.org/z/jKMMbzvYf

    using namespace stdexec;
    using namespace experimental::execution;

    using expected = completion_signatures<set_value_t(bool), set_error_t(int)>;

    auto s = just(true) | then([](bool i) { return !i; }) |
             let_error([](auto&&...) noexcept { return just_error(1); });

    using sc = decltype(get_completion_signatures<decltype(s)>());
    static_assert(std::is_same_v<expected, sc>);

    auto s2 = just(true) | then([](bool i) { return !i; }) | repeat_until() |
              let_error([](auto&&...) noexcept { return just_error(1); });

    using s2c = decltype(get_completion_signatures<decltype(s2)>());
    static_assert(std::is_same_v<expected, s2c>);  // compilation error
    //<source>:21:19: error: static assertion failed due to requirement
    // 'std::is_same_v<stdexec::completion_signatures<stdexec::set_value_t
    // (bool), stdexec::set_error_t (int)>,
    //                       stdexec::_ERROR_<stdexec::dependent_sender_error,
    //                       stdexec::_WITH_SENDER_<stdexec::__basic_sender<stdexec::then_t,
    //                       (lambda at <source>:17:33),
    //                       stdexec::__basic_sender<stdexec::just_t,
    //                       stdexec::__tup::__tuple<bool>>::type>::type>>>'
    //   21 |     static_assert(std::is_same_v<expected, s2c>);
    //      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    // 1 error generated.

    return 0;

In case of s using let_error converts all the error types to int, so result completion is the same as expected.

But this didn't work for s2 with repeat_until. The result completion is far more different than expected.