[libc++] Remove _LIBCPP_DISABLE_NODISCARD_EXTENSIONS and refactor the tests (#87094)

This also adds a few tests that were missing.

NOKEYCHECK=True
GitOrigin-RevId: 83bc7b57714dc2f6b33c188f2b95a0025468ba51

Author: philnik777

.clang-format

@@ -44,7 +44,6 @@ AttributeMacros: [
                   '_LIBCPP_NO_SANITIZE',
                   '_LIBCPP_NO_UNIQUE_ADDRESS',
                   '_LIBCPP_NOALIAS',
-                  '_LIBCPP_NODISCARD_EXT',
                   '_LIBCPP_NODISCARD',
                   '_LIBCPP_NORETURN',
                   '_LIBCPP_OVERRIDABLE_FUNC_VIS',

docs/ReleaseNotes/19.rst

@@ -79,6 +79,10 @@ Deprecations and Removals
   in language modes prior to C++20. If you are using these features prior to C++20, please update to ``-std=c++20``.
   In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
 
+- ``_LIBCPP_DISABLE_NODISCARD_EXT`` has been removed. ``[[nodiscard]]`` applications are now unconditional.
+  This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>` and `P3162 <https://wg21.link/P3162>`
+  to not use ``[[nodiscard]]`` in the standard.
+
 - TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
   it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
   the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in

docs/UsingLibcxx.rst

@@ -196,10 +196,6 @@ safety annotations.
   replacement scenarios from working, e.g. replacing `operator new` and
   expecting a non-replaced `operator new[]` to call the replaced `operator new`.
 
-**_LIBCPP_DISABLE_NODISCARD_EXT**:
-  This macro disables library-extensions of ``[[nodiscard]]``.
-  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>` for more information.
-
 **_LIBCPP_DISABLE_DEPRECATION_WARNINGS**:
   This macro disables warnings when using deprecated components. For example,
   using `std::auto_ptr` when compiling in C++11 mode will normally trigger a
@@ -279,29 +275,6 @@ Libc++ Extensions
 This section documents various extensions provided by libc++, how they're
 provided, and any information regarding how to use them.
 
-.. _nodiscard extension:
-
-Extended applications of ``[[nodiscard]]``
-------------------------------------------
-
-The ``[[nodiscard]]`` attribute is intended to help users find bugs where
-function return values are ignored when they shouldn't be. After C++17 the
-C++ standard has started to declared such library functions as ``[[nodiscard]]``.
-However, this application is limited and applies only to dialects after C++17.
-Users who want help diagnosing misuses of STL functions may desire a more
-liberal application of ``[[nodiscard]]``.
-
-For this reason libc++ provides an extension that does just that! The
-extension is enabled by default and can be disabled by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.
-The extended applications of ``[[nodiscard]]`` takes two forms:
-
-1. Backporting ``[[nodiscard]]`` to entities declared as such by the
-   standard in newer dialects, but not in the present one.
-
-2. Extended applications of ``[[nodiscard]]``, at the library's discretion,
-   applied to entities never declared as such by the standard. You can find
-   all such applications by grepping for ``_LIBCPP_NODISCARD_EXT``.
-
 Extended integral type support
 ------------------------------
 

include/__algorithm/adjacent_find.h

@@ -26,7 +26,7 @@ _LIBCPP_PUSH_MACROS
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Iter, class _Sent, class _BinaryPredicate>
-_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Iter
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _Iter
 __adjacent_find(_Iter __first, _Sent __last, _BinaryPredicate&& __pred) {
   if (__first == __last)
     return __first;
@@ -40,13 +40,13 @@ __adjacent_find(_Iter __first, _Sent __last, _BinaryPredicate&& __pred) {
 }
 
 template <class _ForwardIterator, class _BinaryPredicate>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
 adjacent_find(_ForwardIterator __first, _ForwardIterator __last, _BinaryPredicate __pred) {
   return std::__adjacent_find(std::move(__first), std::move(__last), __pred);
 }
 
 template <class _ForwardIterator>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _ForwardIterator
 adjacent_find(_ForwardIterator __first, _ForwardIterator __last) {
   return std::adjacent_find(std::move(__first), std::move(__last), __equal_to());
 }

include/__algorithm/all_of.h

@@ -19,7 +19,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _InputIterator, class _Predicate>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 all_of(_InputIterator __first, _InputIterator __last, _Predicate __pred) {
   for (; __first != __last; ++__first)
     if (!__pred(*__first))

include/__algorithm/any_of.h

@@ -19,7 +19,7 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _InputIterator, class _Predicate>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 any_of(_InputIterator __first, _InputIterator __last, _Predicate __pred) {
   for (; __first != __last; ++__first)
     if (__pred(*__first))

include/__algorithm/binary_search.h

@@ -22,14 +22,14 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _ForwardIterator, class _Tp, class _Compare>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 binary_search(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value, _Compare __comp) {
   __first = std::lower_bound<_ForwardIterator, _Tp, __comp_ref_type<_Compare> >(__first, __last, __value, __comp);
   return __first != __last && !__comp(__value, *__first);
 }
 
 template <class _ForwardIterator, class _Tp>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 bool
 binary_search(_ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) {
   return std::binary_search(__first, __last, __value, __less<>());
 }

include/__algorithm/clamp.h

@@ -21,7 +21,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17
 template <class _Tp, class _Compare>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&
 clamp(_LIBCPP_LIFETIMEBOUND const _Tp& __v,
       _LIBCPP_LIFETIMEBOUND const _Tp& __lo,
       _LIBCPP_LIFETIMEBOUND const _Tp& __hi,
@@ -31,7 +31,7 @@ clamp(_LIBCPP_LIFETIMEBOUND const _Tp& __v,
 }
 
 template <class _Tp>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&
+[[nodiscard]] inline _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&
 clamp(_LIBCPP_LIFETIMEBOUND const _Tp& __v,
       _LIBCPP_LIFETIMEBOUND const _Tp& __lo,
       _LIBCPP_LIFETIMEBOUND const _Tp& __hi) {

include/__algorithm/count.h

@@ -79,7 +79,7 @@ __count(__bit_iterator<_Cp, _IsConst> __first, __bit_iterator<_Cp, _IsConst> __l
 }
 
 template <class _InputIterator, class _Tp>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __iter_diff_t<_InputIterator>
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __iter_diff_t<_InputIterator>
 count(_InputIterator __first, _InputIterator __last, const _Tp& __value) {
   __identity __proj;
   return std::__count<_ClassicAlgPolicy>(__first, __last, __value, __proj);

include/__algorithm/count_if.h

@@ -20,9 +20,9 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _InputIterator, class _Predicate>
-_LIBCPP_NODISCARD_EXT inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
-    typename iterator_traits<_InputIterator>::difference_type
-    count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred) {
+_LIBCPP_NODISCARD inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
+typename iterator_traits<_InputIterator>::difference_type
+count_if(_InputIterator __first, _InputIterator __last, _Predicate __pred) {
   typename iterator_traits<_InputIterator>::difference_type __r(0);
   for (; __first != __last; ++__first)
     if (__pred(*__first))