Error completion signature of let_error changed recently

[dkolsen-pgi]

This test program started getting compilation errors about five days ago.

#include <stdexec/execution.hpp>

int main() {
  auto snd = stdexec::just_error(2) |
             stdexec::let_error([](int n) noexcept {
                 return stdexec::just_error(n * 10);
             }) |
             stdexec::upon_error([](int n) noexcept { return n + 3; });
  auto opt = stdexec::sync_wait(std::move(snd));
  assert(opt.has_value());
  assert(std::get<0>(*opt) == 23);
}

When compiled with nvc++ 26.5 with the latest stdexec main branch:

$ nvc++ --c++26 --no-stdexec -isystem /proj/cuda/stdexec/main/include letg.cpp
"/proj/cuda/stdexec/main/include/stdexec/__detail/__upon_error.hpp", line 213: error: the deduced placeholder type "stdexec::__sexpr_t<stdexec::upon_error_t, lambda [](int)->int noexcept, stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>>" (aka "stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>") failed the type constraint
      constexpr auto operator()(_Sender&& __sndr, _Fun __fun) const -> __well_formed_sender auto
                                                                       ^
"/proj/cuda/stdexec/main/include/stdexec/__detail/__senders.hpp", line 75: note: concept is false for arguments "<stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>>"
    concept __well_formed_sender = sender_in<_Sender> || dependent_sender<_Sender>;
                                                         ^
"/proj/cuda/stdexec/main/include/stdexec/__detail/__sender_concepts.hpp", line 245: note: concept is false for arguments "<stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>>"
    concept dependent_sender = sender<_Sender> && __is_dependent_sender<_Sender>;
                                                  ^
"/proj/cuda/stdexec/main/include/stdexec/__detail/__get_completion_signatures.hpp", line 454: note: concept is false for arguments "<stdexec::_ERROR_<stdexec::_WHAT_ (stdexec::_INVALID_EXPRESSION_), stdexec::_WHY_ (stdexec::_FUNCTION_IS_NOT_CALLABLE_WITH_THE_GIVEN_ARGUMENTS_), stdexec::_WHERE_ (stdexec::_IN_ALGORITHM_, stdexec::upon_error_t), stdexec::_WITH_FUNCTION_ (lambda [](int)->int noexcept), stdexec::_WITH_ARGUMENTS_ (std::__exception_ptr::exception_ptr)>, stdexec::dependent_sender_error>"
      __std::derived_from<__completion_signatures_of_t<_Sender>, dependent_sender_error>;
      ^
"/proj/cuda/stdexec/main/include/stdexec/__detail/__senders.hpp", line 75: note: concept is false for arguments "<stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>>"
    concept __well_formed_sender = sender_in<_Sender> || dependent_sender<_Sender>;
                                   ^
"/proj/cuda/stdexec/main/include/stdexec/__detail/__sender_concepts.hpp", line 181: note: atomic constraint evaluates to false
      && __constant_completion_signatures_v<STDEXEC::get_completion_signatures<_Sender, _Env...>()>;
         ^
"/usr/include/c++/11/concepts", line 67: note: atomic constraint evaluates to false
      concept derived_from = __is_base_of(_Base, _Derived)
                             ^
          detected during instantiation of "auto stdexec::upon_error_t::operator()(_Sender &&, _Fun) const->stdexec::__well_formed_sender auto [with _Sender=stdexec::<unnamed>::__sexpr<lambda [](_Desc)->auto{}>, _Fun=lambda [](int)->int noexcept]" at line 8 of "letg.cpp"

1 error detected in the compilation of "letg.cpp".

Even though the error message is coming from upon_error, it is the behavior of let_error that changed. It looks like the error completion for that let_error is std::execption_ptr when it had been int previously.

I spent a little time trying to find the change that triggered this failure, but it wasn't obvious to me and I stopped looking.