libstdc++: Use _Bind_front_t/_Bind_back_t in bind_front<f>/bind_back<f> [PR122032]

This patch changes the implementation of bind_front<f> and bind_back<f> to
return a _Bind_front_t<_Bind_fn_t<f>, ...> and _Bind_back_t<_Bind_fn_t<f>, ...>
respectively, replacing the previous lambda-based implementation. The prior use
of a lambda caused non-conforming behavior with respect to C++23 [func.require]
p8, which requires that bind_front<f>(s), bind_front<f>(move(s)), and
bind_front<f>(as_const(s)) produce the same type.

Additionally, using specialized structs reduces the size of the resulting functor
in certain scenarios (see PR).

For the zero-argument case, the function still returns a _Bind_fn_t<f>. Since this
type is already a perfect forwarding call wrapper, it yields the same result as
_Bind_front_t<_Bind_fn_t<f>>.

A consequence of this change is that the types returned by bind_front<f>(args...)
and bind_back<f>(args...) are no longer structural - they are not required to be
structural by the standard.

	PR libstdc++/122032

libstdc++-v3/ChangeLog:

	* include/std/functional (std::bind_front<f>, std::bind_back<f>):
	Define in terms of _Bind_front_t/_Bind_back_t.
	* testsuite/20_util/function_objects/bind_back/nttp.cc: New tests.
	* testsuite/20_util/function_objects/bind_front/nttp.cc: New tests.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>

3 files changed, 45 insertions, 47 deletions

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index dd1aa204eae..1928a27d3fd 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -957,7 +957,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
 #if __cpp_lib_bind_front >= 202306L
-
   /** Create call wrapper by partial application of arguments to function.
    *
    * The result of `std::bind_front<fn>(bind_args...)` is a function object
@@ -970,32 +969,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<auto __fn, typename... _BindArgs>
     constexpr decltype(auto)
     bind_front(_BindArgs&&... __bind_args)
-      noexcept(__and_v<is_nothrow_constructible<_BindArgs>...>)
+    noexcept(__and_v<is_nothrow_constructible<_BindArgs>...>)
     {
       using _Fn = decltype(__fn);
-      static_assert(
-	(is_constructible_v<decay_t<_BindArgs>, _BindArgs> && ...) &&
-	(is_move_constructible_v<decay_t<_BindArgs>> && ...));
       if constexpr (is_pointer_v<_Fn> || is_member_pointer_v<_Fn>)
 	static_assert(__fn != nullptr);
 
       if constexpr (sizeof...(_BindArgs) == 0)
 	return _Bind_fn_t<__fn>{};
-      else {
-	return [... __bound_args(std::forward<_BindArgs>(__bind_args))]
-	  <typename _Self, typename... _CallArgs>
-	  (this _Self&&, _CallArgs&&... __call_args)
-	    noexcept(is_nothrow_invocable_v<
-	      const _Fn&, __like_t<_Self, decay_t<_BindArgs>>..., _CallArgs...>)
-	    -> decltype(auto)
-	    requires is_invocable_v<
-	      const _Fn&, __like_t<_Self, decay_t<_BindArgs>>..., _CallArgs...>
-	  {
-	    return std::invoke(__fn,
-	      std::forward_like<_Self>(__bound_args)...,
-	      std::forward<_CallArgs>(__call_args)...);
-	  };
-      }
+      else
+        return _Bind_front_t<_Bind_fn_t<__fn>, _BindArgs...>(0,
+		 _Bind_fn_t<__fn>{}, std::forward<_BindArgs>(__bind_args)...);
     }
 
 #endif // __cpp_lib_bind_front  // C++26
@@ -1035,36 +1019,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<auto __fn, typename... _BindArgs>
     constexpr decltype(auto)
     bind_back(_BindArgs&&... __bind_args)
-      noexcept(__and_v<is_nothrow_constructible<_BindArgs>...>)
+    noexcept(__and_v<is_nothrow_constructible<_BindArgs>...>)
     {
       using _Fn = decltype(__fn);
-      static_assert(
-	(is_constructible_v<decay_t<_BindArgs>, _BindArgs> && ...) &&
-	(is_move_constructible_v<decay_t<_BindArgs>> && ...));
       if constexpr (is_pointer_v<_Fn> || is_member_pointer_v<_Fn>)
 	static_assert(__fn != nullptr);
 
       if constexpr (sizeof...(_BindArgs) == 0)
 	return _Bind_fn_t<__fn>{};
       else
-      {
-	// Capture arguments in a lambda and return that.
-	return [... __bound_args(std::forward<_BindArgs>(__bind_args))]
-	  <typename _Self, typename... _CallArgs>
-	  (this _Self&&, _CallArgs&&... __call_args)
-	    noexcept(is_nothrow_invocable_v<
-	      const _Fn&, _CallArgs..., __like_t<_Self, decay_t<_BindArgs>>...>)
-	    -> decltype(auto)
-	    requires is_invocable_v<
-	      const _Fn&, _CallArgs..., __like_t<_Self, decay_t<_BindArgs>>...>
-	  {
-	    return std::invoke(__fn,
-	      std::forward<_CallArgs>(__call_args)...,
-	      std::forward_like<_Self>(__bound_args)...);
-	  };
-      }
+        return _Bind_back_t<_Bind_fn_t<__fn>, _BindArgs...>(0,
+		 _Bind_fn_t<__fn>{}, std::forward<_BindArgs>(__bind_args)...);
     }
-
 #endif // __cpp_lib_bind_back  // C++26, nttp
 #endif // __cpp_lib_bind_back
 
@@ -1168,7 +1134,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
 #if __cpp_lib_not_fn >= 202306L
-
   /** Wrap a function type to create a function object that negates its result.
    *
    * The function template `std::not_fn` creates a "forwarding call wrapper",
@@ -1196,7 +1161,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    !std::invoke(__fn, std::forward<_Args>(__args)...); }
 	{ return !std::invoke(__fn, std::forward<_Args>(__args)...); };
     };
-
 #endif // __cpp_lib_not_fn >= 202306L
 #endif // __cpp_lib_not_fn
 
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/nttp.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/nttp.cc
index 3bea8eced43..24bf0466e2d 100644
--- a/libstdc++-v3/testsuite/20_util/function_objects/bind_back/nttp.cc
+++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_back/nttp.cc
@@ -24,6 +24,25 @@ test01()
   struct F { void operator()(int) {} };
   constexpr F f{};
 
+  // Arguments should be decayed:
+  static_assert(std::is_same_v<
+      decltype(bind_back<f>(std::declval<int>())),
+      decltype(bind_back<f>(std::declval<int&>()))
+      >);
+  static_assert(std::is_same_v<
+      decltype(bind_back<f>(std::declval<int>())),
+      decltype(bind_back<f>(std::declval<const int&>()))
+      >);
+
+  static_assert(std::is_same_v<
+      decltype(bind_back<f>(std::declval<int>(), std::declval<float>())),
+      decltype(bind_back<f>(std::declval<int&>(), std::declval<float&>()))
+      >);
+  static_assert(std::is_same_v<
+      decltype(bind_back<f>(std::declval<int>(), std::declval<float>())),
+      decltype(bind_back<f>(std::declval<const int&>(), std::declval<const float&>()))
+      >);
+
   // Reference wrappers should be handled:
   static_assert(!std::is_same_v<
       decltype(bind_back<f>(std::declval<int&>())),
@@ -270,10 +289,8 @@ test04()
   VERIFY(bind_back<g>(1)(2, 3) == 3*1 + 1*2 + 2*3 );
   constexpr auto g2 = bind_back<f>(1, 2);
   VERIFY(g2(3) == 2*1 + 3*2 + 1*3 );
-  VERIFY(bind_back<g1>(2)(3) == 3*1 + 2*2 + 1*3 );
   constexpr auto g3 = bind_back<f>(1, 2, 3);
   VERIFY(g3() == 1 + 2*2 + 3*3);
-  VERIFY(bind_back<g2>(3)() == 1*2 + 2*3 + 3*1 );
   return true;
 }
 
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/nttp.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/nttp.cc
index 9eb3c432a86..cf84353887b 100644
--- a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/nttp.cc
+++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/nttp.cc
@@ -24,6 +24,25 @@ test01()
   struct F { void operator()(int) {} };
   constexpr F f{};
 
+  // Arguments should be decayed:
+  static_assert(std::is_same_v<
+      decltype(bind_front<f>(std::declval<int>())),
+      decltype(bind_front<f>(std::declval<int&>()))
+      >);
+  static_assert(std::is_same_v<
+      decltype(bind_front<f>(std::declval<int>())),
+      decltype(bind_front<f>(std::declval<const int&>()))
+      >);
+
+  static_assert(std::is_same_v<
+      decltype(bind_front<f>(std::declval<int>(), std::declval<float>())),
+      decltype(bind_front<f>(std::declval<int&>(), std::declval<float&>()))
+      >);
+  static_assert(std::is_same_v<
+      decltype(bind_front<f>(std::declval<int>(), std::declval<float>())),
+      decltype(bind_front<f>(std::declval<const int&>(), std::declval<const float&>()))
+      >);
+
   // Reference wrappers should be handled:
   static_assert(!std::is_same_v<
       decltype(bind_front<f>(std::declval<int&>())),
@@ -269,10 +288,8 @@ test04()
   VERIFY( bind_front<g>(1)(2, 3) == 1 + 2*2 + 3*3 );
   constexpr auto g2 = bind_front<f>(1, 2);
   VERIFY( g2(3) == 1 + 2*2 + 3*3 );
-  VERIFY( bind_front<g1>(2)(3) == 1 + 2*2 + 3*3 );
   constexpr auto g3 = bind_front<f>(1, 2, 3);
   VERIFY( g3() == 1 + 2*2 + 3*3 );
-  VERIFY(bind_front<g2>(3)() == 1 + 2*2 + 3*3 );
   return true;
 }