get_env.h

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#pragma once
 
#include "di/execution/concepts/queryable.h"
#include "di/execution/query/forwarding_query.h"
#include "di/execution/types/empty_env.h"
#include "di/function/tag_invoke.h"
#include "di/meta/util.h"
 
namespace di::execution {
namespace detail {
    struct GetEnvFunction {
        template<typename T>
        constexpr auto operator()(T const& value) const -> decltype(auto) {
            if constexpr (concepts::TagInvocable<GetEnvFunction, T const&>) {
                static_assert(concepts::Queryable<meta::TagInvokeResult<GetEnvFunction, T const&>>,
                              "get_env() must return a Queryable.");
                return function::tag_invoke(*this, value);
            } else {
                return types::EmptyEnv {};
            }
        }
 
        constexpr auto operator()() const;
    };
}
 
constexpr inline auto get_env = detail::GetEnvFunction {};
}