Return different* function object when sizeof...(_BoundArgs) == 0

* Such that `std::is_empty_v<F>` is `true` and `operator()` is static.

Author: JMazurkiewicz

libcxx/include/__functional/bind_front.h

@@ -70,6 +70,16 @@ struct __nttp_bind_front_t<_Fn, index_sequence<_Indices...>, _BoundArgs...> {
   }
 };
 
+template <auto _Fn>
+struct __nttp_bind_without_bound_args_t {
+  template <class... _Args>
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Args&&... __args) 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... _Args>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto bind_front(_Args&&... __args) {
   static_assert((is_constructible_v<decay_t<_Args>, _Args> && ...),
@@ -79,8 +89,11 @@ template <auto _Fn, class... _Args>
   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, index_sequence_for<_Args...>, decay_t<_Args>...>{
-      .__bound_args_{std::forward<_Args>(__args)...}};
+  if constexpr (sizeof...(_Args) == 0)
+    return __nttp_bind_without_bound_args_t<_Fn>{};
+  else
+    return __nttp_bind_front_t<_Fn, index_sequence_for<_Args...>, decay_t<_Args>...>{
+        .__bound_args_{std::forward<_Args>(__args)...}};
 }
 
 #endif // _LIBCPP_STD_VER >= 26

libcxx/test/libcxx/utilities/function.objects/func.bind.partial/bind_front.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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// <functional>
+
+// Type of `std::bind_front<NTTP>(/* no bound args */)` is empty.
+
+#include <functional>
+#include <type_traits>
+
+struct NonEmptyFunctionObject {
+  int val = true;
+  void operator()() const;
+};
+
+void func();
+
+struct SomeClass {
+  long member_object;
+  void member_function();
+};
+
+using ResultWithEmptyFuncObject = decltype(std::bind_front<std::integral_constant<int, 0>{}>());
+static_assert(std::is_empty_v<ResultWithEmptyFuncObject>);
+
+using ResultWithNotEmptyFuncObject = decltype(std::bind_front<NonEmptyFunctionObject{}>());
+static_assert(std::is_empty_v<ResultWithNotEmptyFuncObject>);
+
+using ResultWithFunctionPointer = decltype(std::bind_front<func>());
+static_assert(std::is_empty_v<ResultWithFunctionPointer>);
+
+using ResultWithMemberObjectPointer = decltype(std::bind_front<&SomeClass::member_object>());
+static_assert(std::is_empty_v<ResultWithMemberObjectPointer>);
+
+using ResultWithMemberFunctionPointer = decltype(std::bind_front<&SomeClass::member_function>());
+static_assert(std::is_empty_v<ResultWithMemberFunctionPointer>);