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// { dg-do compile { target c++17 } }
#include <utility>
#include <tuple>
#include <functional>
#define LIFT_FWD(x) std::forward<decltype(x)>(x)
template <typename T>
inline
constexpr
auto
equal(
T &&t)
{
return [t = std::forward<T>(t)](const auto& obj)
-> decltype(obj == t)
{
return obj == t;
};
}
template <typename F, typename T>
struct is_tuple_invocable;
template <typename F, typename ... Ts>
struct is_tuple_invocable<F, std::tuple<Ts...>>
{
using type = typename std::is_invocable<F, Ts...>::type;
};
template <typename F>
inline
constexpr
auto
compose(
F&& f
)
noexcept
-> F
{
return std::forward<F>(f);
}
namespace detail {
template <typename F, typename Tail, typename ... T>
inline
constexpr
auto
compose(
std::true_type,
F&& f,
Tail&& tail,
T&& ... objs)
noexcept(noexcept(f(tail(std::forward<T>(objs)...))))
-> decltype(f(tail(std::forward<T>(objs)...)))
{
return f(tail(std::forward<T>(objs)...));
}
}
template <typename F, typename ... Fs>
inline
constexpr
auto
compose(
F&& f,
Fs&&... fs)
{
return [f = std::forward<F>(f), tail = compose(std::forward<Fs>(fs)...)]
(auto&& ... objs)
-> decltype(detail::compose(typename std::is_invocable<decltype(compose(std::forward<Fs>(fs)...)), decltype(objs)...>::type{},
f,
compose(std::forward<Fs>(fs)...),
LIFT_FWD(objs)...))
{
using tail_type = decltype(compose(std::forward<Fs>(fs)...));
#ifndef NOT_VIA_TUPLE
using args_type = std::tuple<decltype(objs)...>;
constexpr auto unitail = typename is_tuple_invocable<tail_type, args_type>::type{};
#else
constexpr auto unitail = typename std::is_invocable<tail_type, decltype(objs)...>::type{};
#endif
return detail::compose(unitail, f, tail, LIFT_FWD(objs)...);
};
}
template <auto N>
constexpr auto eq = equal(N);
static_assert(compose(eq<3>,
std::plus<>{})(1,2),
"compose is constexpr");
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