`sender_in` fails with GCC 13.1 due to `const completion_signatures` in `__constant_completion_signatures_v`

Summary

A sender that compiles successfully with GCC 11.4 fails to compile with GCC 13.1 when passed to exec::start_detached.

The failure appears to come from the sender_in constraint. GCC 13.1 evaluates:

__constant_completion_signatures_v<STDEXEC::get_completion_signatures<_Sender, _Env...>()>

as false, apparently because decltype(_Completions) is const completion_signatures<...>.

GCC 11 succeeds because stdexec has a GCC < 13 workaround that removes const:

std::remove_const_t<decltype(_Completions)>

GCC 13 takes the other branch and does not remove const.

Reproducer

// reproducer.cpp
#include <stdexec/execution.hpp>
using S = decltype(stdexec::just());
static_assert(stdexec::sender_in<S, stdexec::__root_env>);
int main(){}

/* my usage:
#include <stdexec/execution.hpp>
#include <exec/start_detached.hpp>

int main(){
    stdexec::inline_scheduler sch;
    auto sender = stdexec::schedule(sch) | stdexec::then([]() noexcept {  });
    exec::start_detached(std::move(sender));
}
*/

cmake_minimum_required(VERSION 3.21)
project(issue_stdexec 
    VERSION 0.0.1 
    LANGUAGES CXX)

include(CPM.cmake)
CPMAddPackage(
  NAME stdexec
  GITHUB_REPOSITORY NVIDIA/stdexec
  GIT_TAG 6d7ad68
)

add_executable(reproducer reproducer.cpp)
target_link_libraries(reproducer PUBLIC STDEXEC::stdexec)

Environment

Linux GCC 11.4: compiles successfully
Linux GCC 13.1: fails
C++ mode: -std=gnu++20

Possible Cause

The issue may be related to stdexec/__detail/__sender_concepts.hpp #2027 :

#if STDEXEC_GCC() && STDEXEC_GCC_VERSION < 1300
  template <auto _Completions>
  inline constexpr bool __constant_completion_signatures_v =
    __valid_completion_signatures<std::remove_const_t<decltype(_Completions)>>;
#else
  template <auto _Completions>
  inline constexpr bool __constant_completion_signatures_v =
    __valid_completion_signatures<decltype(_Completions)>;
#endif

On GCC 13.1, the remove_const_t workaround still seems necessary.

Start with stdexec/__detail/__sender_concepts.hpp, especially __constant_completion_signatures_v and its GCC version branches. Build the provided reproducer.cpp with GCC 11.4 and GCC 13.1 under C++20, then trace the sender_in constraint evaluation. Done means the reproducer compiles on GCC 13.1 without regressing the older compiler path.