[libc++][functional] Implement not_fn<NTTP>

libcxx/docs/FeatureTestMacroTable.rst

@@ -458,6 +458,8 @@ Status
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_mdspan``                                       ``202406L``
     ---------------------------------------------------------- -----------------
+    ``__cpp_lib_not_fn``                                       ``202306L``
+    ---------------------------------------------------------- -----------------
     ``__cpp_lib_optional_range_support``                       *unimplemented*
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_out_ptr``                                      ``202311L``

libcxx/docs/Status/Cxx2cPapers.csv

@@ -24,7 +24,7 @@
 "`P1383R2 <https://wg21.link/P1383R2>`__","More ``constexpr`` for ``<cmath>`` and ``<complex>``","2023-06 (Varna)","","",""
 "`P2734R0 <https://wg21.link/P2734R0>`__","Adding the new SI prefixes","2023-06 (Varna)","|Complete|","17",""
 "`P2548R6 <https://wg21.link/P2548R6>`__","``copyable_function``","2023-06 (Varna)","","",""
-"`P2714R1 <https://wg21.link/P2714R1>`__","Bind front and back to NTTP callables","2023-06 (Varna)","","",""
+"`P2714R1 <https://wg21.link/P2714R1>`__","Bind front and back to NTTP callables","2023-06 (Varna)","|Partial|","20","``not_fn`` only"
 "`P2630R4 <https://wg21.link/P2630R4>`__","``submdspan``","2023-06 (Varna)","","",""
 "","","","","",""
 "`P0543R3 <https://wg21.link/P0543R3>`__","Saturation arithmetic","2023-11 (Kona)","|Complete|","18",""

libcxx/include/__functional/not_fn.h

@@ -16,6 +16,8 @@
 #include <__type_traits/decay.h>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_constructible.h>
+#include <__type_traits/is_member_pointer.h>
+#include <__type_traits/is_pointer.h>
 #include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -48,6 +50,27 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 auto not_fn(_Fn&& __f) {
 
 #endif // _LIBCPP_STD_VER >= 17
 
+#if _LIBCPP_STD_VER >= 26
+
+template <auto _Fn>
+struct __nttp_not_fn_t {
+  template <class... _Args>
+  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Args&&... __args) const
+      noexcept(noexcept(!std::invoke(_Fn, std::forward<_Args>(__args)...)))
+          -> decltype(!std::invoke(_Fn, std::forward<_Args>(__args)...)) {
+    return !std::invoke(_Fn, std::forward<_Args>(__args)...);
+  }
+};
+
+template <auto _Fn>
+[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr auto not_fn() noexcept {
+  if constexpr (using _Ty = decltype(_Fn); is_pointer_v<_Ty> || is_member_pointer_v<_Ty>)
+    static_assert(_Fn != nullptr, "f cannot be equal to nullptr");
+  return __nttp_not_fn_t<_Fn>();
+}
+
+#endif // _LIBCPP_STD_VER >= 26
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FUNCTIONAL_NOT_FN_H

libcxx/include/functional

@@ -214,7 +214,9 @@ template <class Predicate> // deprecated in C++17, removed in C++20
 binary_negate<Predicate> not2(const Predicate& pred);
 
 template <class F>
-constexpr unspecified not_fn(F&& f); // C++17, constexpr in C++20
+  constexpr unspecified not_fn(F&& f);     // C++17, constexpr in C++20
+template <auto f>
+  constexpr unspecified not_fn() noexcept; // C++26
 
 // [func.bind.partial], function templates bind_front and bind_back
 template<class F, class... Args>

libcxx/include/version

@@ -171,7 +171,8 @@ __cpp_lib_nonmember_container_access                    201411L <array> <deque>
                                                                 <iterator> <list> <map>
                                                                 <regex> <set> <string>
                                                                 <unordered_map> <unordered_set> <vector>
-__cpp_lib_not_fn                                        201603L <functional>
+__cpp_lib_not_fn                                        202306L <functional>
+                                                        201603L // C++17
 __cpp_lib_null_iterators                                201304L <iterator>
 __cpp_lib_optional                                      202110L <optional>
                                                         202106L // C++20
@@ -557,6 +558,8 @@ __cpp_lib_void_t                                        201411L <type_traits>
 // # define __cpp_lib_linalg                               202311L
 # undef  __cpp_lib_mdspan
 # define __cpp_lib_mdspan                               202406L
+# undef  __cpp_lib_not_fn
+# define __cpp_lib_not_fn                               202306L
 // # define __cpp_lib_optional_range_support               202406L
 # undef  __cpp_lib_out_ptr
 # define __cpp_lib_out_ptr                              202311L

libcxx/test/libcxx/utilities/function.objects/func.not.fn/not_fn.nttp.compile.pass.cpp

@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <functional>
+
+// Type of `std::not_fn<NTTP>()` is always empty.
+
+#include <functional>
+#include <type_traits>
+
+struct NonEmptyFunctionObject {
+  bool val = true;
+  bool operator()() const;
+};
+
+bool func();
+
+struct SomeClass {
+  bool member_object;
+  bool member_function();
+};
+
+using ResultWithEmptyFuncObject = decltype(std::not_fn<std::false_type{}>());
+static_assert(std::is_empty_v<ResultWithEmptyFuncObject>);
+
+using ResultWithNotEmptyFuncObject = decltype(std::not_fn<NonEmptyFunctionObject{}>());
+static_assert(std::is_empty_v<ResultWithNotEmptyFuncObject>);
+
+using ResultWithFunctionPointer = decltype(std::not_fn<&func>());
+static_assert(std::is_empty_v<ResultWithFunctionPointer>);
+
+using ResultWithMemberObjectPointer = decltype(std::not_fn<&SomeClass::member_object>());
+static_assert(std::is_empty_v<ResultWithMemberObjectPointer>);
+
+using ResultWithMemberFunctionPointer = decltype(std::not_fn<&SomeClass::member_function>());
+static_assert(std::is_empty_v<ResultWithMemberFunctionPointer>);

libcxx/test/libcxx/utilities/function.objects/func.not.fn/not_fn.nttp.nodiscard.verify.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20, c++23
+
+// <functional>
+
+// Test the libc++ extension that std::not_fn<NTTP> is marked as [[nodiscard]].
+
+#include <functional>
+#include <type_traits>
+
+void test() {
+  using F = std::true_type;
+  std::not_fn<F{}>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+  auto negated = std::not_fn<F{}>();
+  negated(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}

libcxx/test/std/language.support/support.limits/support.limits.general/functional.version.compile.pass.cpp

@@ -27,6 +27,7 @@
     __cpp_lib_invoke_r                 202106L [C++23]
     __cpp_lib_move_only_function       202110L [C++23]
     __cpp_lib_not_fn                   201603L [C++17]
+                                       202306L [C++26]
     __cpp_lib_ranges                   202110L [C++20]
                                        202406L [C++23]
     __cpp_lib_reference_wrapper        202403L [C++26]
@@ -525,8 +526,8 @@
 # ifndef __cpp_lib_not_fn
 #   error "__cpp_lib_not_fn should be defined in c++26"
 # endif
-# if __cpp_lib_not_fn != 201603L
-#   error "__cpp_lib_not_fn should have the value 201603L in c++26"
+# if __cpp_lib_not_fn != 202306L
+#   error "__cpp_lib_not_fn should have the value 202306L in c++26"
 # endif
 
 # ifndef __cpp_lib_ranges

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

@@ -156,6 +156,7 @@
     __cpp_lib_node_extract                                  201606L [C++17]
     __cpp_lib_nonmember_container_access                    201411L [C++17]
     __cpp_lib_not_fn                                        201603L [C++17]
+                                                            202306L [C++26]
     __cpp_lib_null_iterators                                201304L [C++14]
     __cpp_lib_optional                                      201606L [C++17]
                                                             202106L [C++20]
@@ -7405,8 +7406,8 @@
 # ifndef __cpp_lib_not_fn
 #   error "__cpp_lib_not_fn should be defined in c++26"
 # endif
-# if __cpp_lib_not_fn != 201603L
-#   error "__cpp_lib_not_fn should have the value 201603L in c++26"
+# if __cpp_lib_not_fn != 202306L
+#   error "__cpp_lib_not_fn should have the value 202306L in c++26"
 # endif
 
 # ifndef __cpp_lib_null_iterators