[gcc r14-3901] libstdc++: Remove std::bind_front specialization for no bound args

[gcc r14-3901] libstdc++: Remove std::bind_front specialization for no bound args

[Patrick Palka]

https://gcc.gnu.org/g:f1e87aee5b7023fb4f5791c6869db705e18c2705

commit r14-3901-gf1e87aee5b7023fb4f5791c6869db705e18c2705
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Sep 12 11:23:08 2023 -0400

    libstdc++: Remove std::bind_front specialization for no bound args
    
    The specialization used by std::bind_front when there are 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.  What's left to consider is the compile time
    advantage of the specialization, which doesn't seem huge since it just
    avoids using tuple<> (which is an explicit specialization anyway) and
    expanding some pack expansions with an empty argument pack.  So this
    patch removes this specialization; this means we have one less spot to
    fix the PR libstdc++/111327 perfect forwarding bug.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/functional (_Bind_front0): Remove.
            (_Bind_front_t): Adjust.

Diff:
---
 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 60d4d1f3dd22..7d1b890bb4e9 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.
    *