[libc++][numeric] Marked saturation artithmetic functions as [[nodiscard]]

...according to Coding Guidelines:

- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
- #166524 (comment)

Author: H-G-Hristov

libcxx/include/__numeric/saturation_arithmetic.h

@@ -121,27 +121,27 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Rp __saturate_cast(_Tp __x) noexcept {
 #if _LIBCPP_STD_VER >= 26
 
 template <__signed_or_unsigned_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp add_sat(_Tp __x, _Tp __y) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp add_sat(_Tp __x, _Tp __y) noexcept {
   return std::__add_sat(__x, __y);
 }
 
 template <__signed_or_unsigned_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp sub_sat(_Tp __x, _Tp __y) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp sub_sat(_Tp __x, _Tp __y) noexcept {
   return std::__sub_sat(__x, __y);
 }
 
 template <__signed_or_unsigned_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp mul_sat(_Tp __x, _Tp __y) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp mul_sat(_Tp __x, _Tp __y) noexcept {
   return std::__mul_sat(__x, __y);
 }
 
 template <__signed_or_unsigned_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp div_sat(_Tp __x, _Tp __y) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp div_sat(_Tp __x, _Tp __y) noexcept {
   return std::__div_sat(__x, __y);
 }
 
 template <__signed_or_unsigned_integer _Rp, __signed_or_unsigned_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Rp saturate_cast(_Tp __x) noexcept {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Rp saturate_cast(_Tp __x) noexcept {
   return std::__saturate_cast<_Rp>(__x);
 }
 

libcxx/test/libcxx/numerics/numeric.ops/sat.nodiscard.verify.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// <numeric>
+
+#include <numeric>
+
+// clang-format off
+void test() {
+  std::add_sat(94, 82);               // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::sub_sat(94, 82);               // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::mul_sat(94, 82);               // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::div_sat(94, 82);               // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  std::saturate_cast<signed int>(49); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+}
+// clang-format on