[libc++] Deprecate and remove std::uncaught_exception (#101830)

Works towards P0619R4/#99985.

- std::uncaught_exception was not previously deprecated. This patch
  deprecates it since C++17 as per N4259. std::uncaught_exceptions is
  used instead as libc++ unconditionally provides this function.

- _LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION restores
  std::uncaught_exception.

- As a drive-by, this patch updates the C++20 status page to 
  explain that D.11 is already done, since it was done in 
  578d09c.

Author: frederick-vs-ja

libcxx/docs/ReleaseNotes/20.rst

@@ -47,6 +47,8 @@ Improvements and New Features
 - The ``lexicographical_compare`` and ``ranges::lexicographical_compare`` algorithms have been optimized for trivially
   equality comparable types, resulting in a performance improvement of up to 40x.
 
+- The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception`` available in C++20 and later modes.
+
 
 Deprecations and Removals
 -------------------------

libcxx/docs/Status/Cxx17.rst

@@ -38,6 +38,7 @@ Paper Status
 
 .. note::
 
+   .. [#note-N4259] N4259: ``std::uncaught_exception`` is deprecated since LLVM 20.0.
    .. [#note-P0067R5] P0067R5: ``std::(to|from)_chars`` for integrals has been available since version 7.0. ``std::to_chars`` for ``float`` and ``double`` since version 14.0 ``std::to_chars`` for ``long double`` uses the implementation for ``double``.
    .. [#note-P0226] P0226: Progress is tracked `here <https://github.com/llvm/llvm-project/issues/99939>`_.
    .. [#note-P0607] P0607: The parts of P0607 that are not done are the ``<regex>`` bits.

libcxx/docs/Status/Cxx17Papers.csv

@@ -4,7 +4,7 @@
 "`N4169 <https://wg21.link/N4169>`__","A proposal to add invoke function template","2014-11 (Urbana)","|Complete|","3.7",""
 "`N4190 <https://wg21.link/N4190>`__","Removing auto_ptr, random_shuffle(), And Old <functional> Stuff.","2014-11 (Urbana)","|Complete|","15.0",""
 "`N4258 <https://wg21.link/N4258>`__","Cleaning-up noexcept in the Library.","2014-11 (Urbana)","|In Progress|","3.7",""
-"`N4259 <https://wg21.link/N4259>`__","Wording for std::uncaught_exceptions","2014-11 (Urbana)","|Complete|","3.7",""
+"`N4259 <https://wg21.link/N4259>`__","Wording for std::uncaught_exceptions","2014-11 (Urbana)","|Complete| [#note-N4259]_","3.7",""
 "`N4277 <https://wg21.link/N4277>`__","TriviallyCopyable ``reference_wrapper``\ .","2014-11 (Urbana)","|Complete|","3.2",""
 "`N4279 <https://wg21.link/N4279>`__","Improved insertion interface for unique-key maps.","2014-11 (Urbana)","|Complete|","3.7",""
 "`N4280 <https://wg21.link/N4280>`__","Non-member size() and more","2014-11 (Urbana)","|Complete|","3.6",""

libcxx/docs/Status/Cxx20.rst

@@ -42,7 +42,7 @@ Paper Status
    .. [#note-P0645] P0645: The implementation was complete since Clang 14,
       except the feature-test macro was not set until Clang 19.
    .. [#note-P0966] P0966: It was previously erroneously marked as complete in version 8.0. See `bug 45368 <https://llvm.org/PR45368>`__.
-   .. [#note-P0619] P0619: Only sections D.8, D.9, D.10 and D.13 are implemented. Sections D.4, D.7, D.11, and D.12 remain undone.
+   .. [#note-P0619] P0619: Only sections D.7, D.8, D.9, D.10, D.11 and D.13 are implemented. Sections D.4 and D.12 remain undone.
    .. [#note-P0883.1] P0883: shared_ptr and floating-point changes weren't applied as they themselves aren't implemented yet.
    .. [#note-P0883.2] P0883: ``ATOMIC_FLAG_INIT`` was marked deprecated in version 14.0, but was undeprecated with the implementation of LWG3659 in version 15.0.
    .. [#note-P0660] P0660: The paper is implemented but the features are experimental and can be enabled via ``-fexperimental-library``.

libcxx/docs/UserDocumentation.rst

@@ -163,6 +163,9 @@ C++17 Specific Configuration Macros
 
 C++20 Specific Configuration Macros
 -----------------------------------
+**_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION**:
+  This macro is used to re-enable `uncaught_exception`.
+
 **_LIBCPP_ENABLE_CXX20_REMOVED_SHARED_PTR_UNIQUE**:
   This macro is used to re-enable the function
   ``std::shared_ptr<...>::unique()``.

libcxx/include/__exception/operations.h

@@ -29,7 +29,9 @@ using terminate_handler = void (*)();
 _LIBCPP_EXPORTED_FROM_ABI terminate_handler set_terminate(terminate_handler) _NOEXCEPT;
 _LIBCPP_EXPORTED_FROM_ABI terminate_handler get_terminate() _NOEXCEPT;
 
-_LIBCPP_EXPORTED_FROM_ABI bool uncaught_exception() _NOEXCEPT;
+#if _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION)
+_LIBCPP_EXPORTED_FROM_ABI _LIBCPP_DEPRECATED_IN_CXX17 bool uncaught_exception() _NOEXCEPT;
+#endif // _LIBCPP_STD_VER <= 17 || defined(_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION)
 _LIBCPP_EXPORTED_FROM_ABI int uncaught_exceptions() _NOEXCEPT;
 
 class _LIBCPP_EXPORTED_FROM_ABI exception_ptr;

libcxx/include/__ostream/basic_ostream.h

@@ -152,7 +152,7 @@ basic_ostream<_CharT, _Traits>::sentry::sentry(basic_ostream<_CharT, _Traits>& _
 
 template <class _CharT, class _Traits>
 basic_ostream<_CharT, _Traits>::sentry::~sentry() {
-  if (__os_.rdbuf() && __os_.good() && (__os_.flags() & ios_base::unitbuf) && !uncaught_exception()) {
+  if (__os_.rdbuf() && __os_.good() && (__os_.flags() & ios_base::unitbuf) && uncaught_exceptions() == 0) {
 #ifndef _LIBCPP_HAS_NO_EXCEPTIONS
     try {
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS

libcxx/include/exception

@@ -47,7 +47,7 @@ terminate_handler set_terminate(terminate_handler  f ) noexcept;
 terminate_handler get_terminate() noexcept;
 [[noreturn]] void terminate() noexcept;
 
-bool uncaught_exception()  noexcept;
+bool uncaught_exception()  noexcept;  // deprecated in C++17, removed in C++20
 int  uncaught_exceptions() noexcept;  // C++17
 
 typedef unspecified exception_ptr;

libcxx/modules/std/exception.inc

@@ -21,6 +21,8 @@ export namespace std {
   using std::terminate;
   using std::terminate_handler;
   using std::throw_with_nested;
+#ifdef _LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION
   using std::uncaught_exception;
+#endif
   using std::uncaught_exceptions;
 } // namespace std

libcxx/src/exception.cpp

@@ -6,6 +6,9 @@
 //
 //===----------------------------------------------------------------------===//
 
+#define _LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION
+#define _LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 #include <exception>
 #include <new>
 #include <typeinfo>

libcxx/test/std/language.support/support.exception/uncaught/uncaught_exception.pass.cpp

@@ -7,6 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: no-exceptions
+
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_DISABLE_DEPRECATION_WARNINGS
+
 // test uncaught_exception
 
 #include <exception>

libcxx/test/std/language.support/support.exception/uncaught/uncaught_exeption.depr_in_cxx17.verify.cpp

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <exception>
+
+// uncaught_exception
+//  deprecated in C++17
+
+// UNSUPPORTED: c++03, c++11, c++14
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION
+
+#include <exception>
+
+void test() {
+  (void)std::uncaught_exception(); // expected-warning {{'uncaught_exception' is deprecated}}
+}