Make modulizable in g++.

[anonymouspc]

Recently I'm trying to compile beman/execution into C++20 Modules:

module;
#include <beman/execution/execution.hpp>
#include <beman/execution/functional.hpp>
#include <beman/execution/stop_token.hpp>
export namespace beman::execution
{
    using beman::execution::sender;
    /*using...*/
}

while g++ complains about one exposes TU-local entity:

error: ‘using std::conditional_t = typename std::conditional<same_as<typename beman::execution::detail::completion_domain_merge<typename beman::execution::detail::completion_domain_merge<decltype (get(beman::execution::set_error)), decltype (get(beman::execution::set_stopped))>::type, decltype (get(beman::execution::set_value))>::type, beman::execution::detail::completion_domain_undefined>, Default, typename beman::execution::detail::completion_domain_merge<typename beman::execution::detail::completion_domain_merge<decltype (get(beman::execution::set_error)), decltype (get(beman::execution::set_stopped))>::type, decltype (get(beman::execution::set_value))>::type>::type’ exposes TU-local entity ‘struct std::conditional<same_as<typename beman::execution::detail::completion_domain_merge<typename beman::execution::detail::completion_domain_merge<decltype (get(beman::execution::set_error)), decltype (get(beman::execution::set_stopped))>::type, decltype (get(beman::execution::set_value))>::type, beman::execution::detail::completion_domain_undefined>, Default, typename beman::execution::detail::completion_domain_merge<typename beman::execution::detail::completion_domain_merge<decltype (get(beman::execution::set_error)), decltype (get(beman::execution::set_stopped))>::type, decltype (get(beman::execution::set_value))>::type>’

in beman/execution/detail/completion_domain.hpp.

When we look into completion_domain.hpp, here

• get is a function-local lambda with local captures (that is to say, is TU-local)
• TU-local entities will indirectly affect all the entities which depends on it to be not module-exportable after P1815. Here, the templates of using type = ...; return std::conditional<...type...>; are infected as TU-local.

So I made a tiny change on get function, making it constexpr and exportable.

Thank you! This is an amazing library (super tidy!!) and I'll star it.

[dietmarkuehl]

This change seems like an improvement, i.e., I'll and it. However, I want to avoid lambdas in headers. Sadly, I didn't realise that when I started implementing the library and there are obviously some still there. I'll change that to use a struct instead.

I also wanted to provide suitable modules declaration. However, when I last tried compilers and/or the build system weren't quite ready.

Thanks!

[coveralls]

coverage: 92.436%. remained the same
when pulling 793f452 on anonymouspc:main
into a5843eb on bemanproject:main.