[libc++][functional] Implement std::bind_front<NTTP>

libcxx/include/__functional/bind_front.h

@@ -17,6 +17,8 @@
 #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)
@@ -49,6 +51,38 @@ _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Fn&& __f, _Args&&... __args) {
 
 #endif // _LIBCPP_STD_VER >= 20
 
+#if _LIBCPP_STD_VER >= 26
+
+template <auto _Fn>
+struct __nttp_bind_front_op {
+  template <class... _Args>
+  _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, class... _BoundArgs>
+struct __nttp_bind_front_t : __perfect_forward<__nttp_bind_front_op<_Fn>, _BoundArgs...> {
+  using __perfect_forward<__nttp_bind_front_op<_Fn>, _BoundArgs...>::__perfect_forward;
+};
+
+template <auto _Fn, class... _Args>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Args&&... __args) {
+  static_assert((is_constructible_v<decay_t<_Args>, _Args> && ...),
+                "bind_front requires all decay_t<Args> to be constructible from respective Args");
+  static_assert((is_move_constructible_v<decay_t<_Args>> && ...),
+                "bind_front requires all decay_t<Args> to be move constructible");
+  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_bind_front_t<_Fn, decay_t<_Args>...>(std::forward<_Args>(__args)...);
+}
+
+#endif // _LIBCPP_STD_VER >= 26
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FUNCTIONAL_BIND_FRONT_H

libcxx/include/functional

@@ -221,6 +221,8 @@ template <auto f>
 // [func.bind.partial], function templates bind_front and bind_back
 template<class F, class... Args>
   constexpr unspecified bind_front(F&&, Args&&...); // C++20
+template<auto f, class... Args>
+  constexpr unspecified bind_front(Args&&...);      // C++26
 template<class F, class... Args>
   constexpr unspecified bind_back(F&&, Args&&...);  // C++23
 

libcxx/test/libcxx/utilities/function.objects/func.bind.partial/bind_front.nttp.nodiscard.verify.cpp

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <functional>
+
+// Test the libc++ extension that std::bind_front<NTTP> is marked as [[nodiscard]].
+
+#include <functional>
+
+void test() {
+  std::bind_front<test>(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}