https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107784
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Complete patch:
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -995,9 +995,68 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
std::tuple<_BoundArgs...> _M_bound_args;
};
+ template<typename _Fd>
+ struct _Bind_front0
+ {
+ static_assert(is_move_constructible_v<_Fd>);
+
+ // First parameter is to ensure this constructor is never used
+ // instead of the copy/move constructor.
+ template<typename _Fn>
+ explicit constexpr
+ _Bind_front0(int, _Fn&& __fn)
+ noexcept(is_nothrow_constructible_v<_Fd, _Fn>)
+ : _M_fd(std::forward<_Fn>(__fn))
+ { }
+
+ _Bind_front0(const _Bind_front0&) = default;
+ _Bind_front0(_Bind_front0&&) = default;
+ _Bind_front0& operator=(const _Bind_front0&) = default;
+ _Bind_front0& operator=(_Bind_front0&&) = default;
+ ~_Bind_front0() = default;
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<_Fd&, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) &
+ noexcept(is_nothrow_invocable_v<_Fd&, _CallArgs...>)
+ { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<const _Fd&, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) const &
+ noexcept(is_nothrow_invocable_v<const _Fd&, _CallArgs...>)
+ { return std::invoke(_M_fd, std::forward<_CallArgs>(__call_args)...); }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<_Fd, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) &&
+ noexcept(is_nothrow_invocable_v<_Fd, _CallArgs...>)
+ {
+ return std::invoke(std::move(_M_fd),
+ std::forward<_CallArgs>(__call_args)...);
+ }
+
+ template<typename... _CallArgs>
+ constexpr
+ invoke_result_t<const _Fd, _CallArgs...>
+ operator()(_CallArgs&&... __call_args) const &&
+ noexcept(is_nothrow_invocable_v<const _Fd, _CallArgs...>)
+ {
+ return std::invoke(std::move(_M_fd),
+ std::forward<_CallArgs>(__call_args)...);
+ }
+
+ private:
+ _Fd _M_fd;
+ };
+
template<typename _Fn, typename... _Args>
using _Bind_front_t
- = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
+ = __conditional_t<sizeof...(_Args) == 0, _Bind_front0<decay_t<_Fn>>,
+ _Bind_front<decay_t<_Fn>, decay_t<_Args>...>>;
/** Create call wrapper by partial application of arguments to function.
*
