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

stdexec/__detail/__sender_concepts.hpp から始め、特に __constant_completion_signatures_v とその GCC バージョン分岐を確認します。提供された reproducer.cpp を GCC 11.4 および GCC 13.1 で C++20 としてビルドし、その後 sender_in 制約の評価を追跡します。reproducer が GCC 13.1 でコンパイルでき、古いコンパイラのパスにリグレッションがないことをもって完了とします。

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.