Re: [PATCH 1/3] libstdc++: Remove std::bind_front specialization for no bound args

[Patrick Palka <ppalka@redhat.com>]

On Mon, 11 Sep 2023, Patrick Palka wrote:

> This specialization for the case of no bound args, added by
> r13-4214-gcbd05ca5ab1231, seems to be mostly obsoleted by
> r13-5033-ge2eab3c4edb6aa which added [[no_unique_address]] to the
> main template's data members.  And the compile time advantage of
> avoiding an empty tuple and index_sequence seems minimal.  Removing this
> specialization also means we don't have to fix the PR111327 bug in
> another place.

FWIW I don't feel strongly about removing this specialization.  If we
keep it We'd at least be able to reuse it for std::bind_back, and it
wouldn't be hard to fix the PR111327 bug in its implementation.

> 
> 	PR libstdc++/111327
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/functional (_Bind_front0): Remove.
> 	(_Bind_front_t): Adjust.
> ---
>  libstdc++-v3/include/std/functional | 63 +----------------------------
>  1 file changed, 1 insertion(+), 62 deletions(-)
> 
> diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
> index 60d4d1f3dd2..7d1b890bb4e 100644
> --- a/libstdc++-v3/include/std/functional
> +++ b/libstdc++-v3/include/std/functional
> @@ -996,69 +996,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        [[no_unique_address]] std::tuple<_BoundArgs...> _M_bound_args;
>      };
>  
> -  // Avoid the overhead of an empty tuple<> if there are no 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:
> -      [[no_unique_address]] _Fd _M_fd;
> -    };
> -
>    template<typename _Fn, typename... _Args>
> -    using _Bind_front_t
> -      = __conditional_t<sizeof...(_Args) == 0, _Bind_front0<decay_t<_Fn>>,
> -			_Bind_front<decay_t<_Fn>, decay_t<_Args>...>>;
> +    using _Bind_front_t = _Bind_front<decay_t<_Fn>, decay_t<_Args>...>;
>  
>    /** Create call wrapper by partial application of arguments to function.
>     *
> -- 
> 2.42.0.158.g94e83dcf5b
> 
>