[libc++] Implement P0627R6 (Function to mark unreachable code)

Reviewed By: ldionne, Quuxplusone, #libc

Spies: arichardson, mstorsjo, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D119152

libcxx/docs/FeatureTestMacroTable.rst

@@ -350,6 +350,6 @@ Status
     ------------------------------------------------- -----------------
     ``__cpp_lib_to_underlying``                       ``202102L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_unreachable``                         *unimplemented*
+    ``__cpp_lib_unreachable``                         ``202202L``
     ================================================= =================
 

libcxx/docs/ReleaseNotes.rst

@@ -38,6 +38,8 @@ What's New in Libc++ 15.0.0?
 New Features
 ------------
 
+ - Implemented P0627R6 (Function to mark unreachable code)
+
 API Changes
 -----------
 

libcxx/docs/Status/Cxx2bPapers.csv

@@ -40,7 +40,7 @@
 "","","","","",""
 "`P0323R12 <https://wg21.link/P0323R12>`__","LWG","``std::expected``","February 2022","",""
 "`P0533R9 <https://wg21.link/P0533R9>`__","LWG","``constexpr`` for ``<cmath>`` and ``<cstdlib>``","February 2022","",""
-"`P0627R6 <https://wg21.link/P0627R6>`__","LWG","Function to mark unreachable code","February 2022","",""
+"`P0627R6 <https://wg21.link/P0627R6>`__","LWG","Function to mark unreachable code","February 2022","|Complete|","15.0"
 "`P1206R7 <https://wg21.link/P1206R7>`__","LWG","``ranges::to``: A function to convert any range to a container","February 2022","",""
 "`P1413R3 <https://wg21.link/P1413R3>`__","LWG","Deprecate ``std::aligned_storage`` and ``std::aligned_union``","February 2022","",""
 "`P2255R3 <https://wg21.link/P2255R3>`__","LWG","A type trait to detect reference binding to temporary","February 2022","",""

libcxx/include/CMakeLists.txt

@@ -401,6 +401,7 @@ set(files
   __utility/swap.h
   __utility/to_underlying.h
   __utility/transaction.h
+  __utility/unreachable.h
   __variant/monostate.h
   algorithm
   any

libcxx/include/__filesystem/directory_entry.h

@@ -20,6 +20,7 @@
 #include <__filesystem/operations.h>
 #include <__filesystem/path.h>
 #include <__filesystem/perms.h>
+#include <__utility/unreachable.h>
 #include <chrono>
 #include <cstdint>
 #include <cstdlib>
@@ -362,7 +363,7 @@ private:
         __ec->clear();
       return __data_.__type_;
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -383,7 +384,7 @@ private:
       return __data_.__type_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -398,7 +399,7 @@ private:
     case _RefreshSymlink:
       return file_status(__get_ft(__ec), __data_.__non_sym_perms_);
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -414,7 +415,7 @@ private:
     case _RefreshSymlinkUnresolved:
       return file_status(__get_sym_ft(__ec), __data_.__sym_perms_);
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -439,7 +440,7 @@ private:
       return __data_.__size_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -458,7 +459,7 @@ private:
       return __data_.__nlink_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -481,7 +482,7 @@ private:
       return __data_.__write_time_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
 private:

libcxx/include/__format/format_arg.h

@@ -16,6 +16,7 @@
 #include <__format/format_fwd.h>
 #include <__format/format_parse_context.h>
 #include <__memory/addressof.h>
+#include <__utility/unreachable.h>
 #include <__variant/monostate.h>
 #include <string>
 #include <string_view>
@@ -77,7 +78,7 @@ visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg) {
 #ifndef _LIBCPP_HAS_NO_INT128
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__i128);
 #else
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
 #endif
   case __format::__arg_t::__unsigned:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__unsigned);
@@ -88,7 +89,7 @@ visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg) {
 #ifndef _LIBCPP_HAS_NO_INT128
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__u128);
 #else
-   _LIBCPP_UNREACHABLE();
+   __libcpp_unreachable();
 #endif
   case __format::__arg_t::__float:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__float);
@@ -106,7 +107,7 @@ visit_format_arg(_Visitor&& __vis, basic_format_arg<_Context> __arg) {
   case __format::__arg_t::__handle:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__handle);
   }
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 template <class _Context>

libcxx/include/__format/formatter.h

@@ -18,6 +18,7 @@
 #include <__format/format_fwd.h>
 #include <__format/format_string.h>
 #include <__format/parser_std_format_spec.h>
+#include <__utility/unreachable.h>
 #include <string_view>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -116,7 +117,7 @@ __padding_size(size_t __size, size_t __width,
   size_t __fill = __width - __size;
   switch (__align) {
   case __format_spec::_Flags::_Alignment::__default:
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
 
   case __format_spec::_Flags::_Alignment::__left:
     return {0, __fill};
@@ -132,7 +133,7 @@ __padding_size(size_t __size, size_t __width,
   case __format_spec::_Flags::_Alignment::__right:
     return {__fill, 0};
   }
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 /**

libcxx/include/__format/formatter_floating_point.h

@@ -27,6 +27,7 @@
 #include <__format/formatter_integral.h>
 #include <__format/parser_std_format_spec.h>
 #include <__utility/move.h>
+#include <__utility/unreachable.h>
 #include <charconv>
 #include <cmath>
 
@@ -689,7 +690,7 @@ private:
 
     default:
       _LIBCPP_ASSERT(false, "The parser should have validated the type");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
   }
 };

libcxx/include/__format/formatter_integral.h

@@ -19,6 +19,7 @@
 #include <__format/format_fwd.h>
 #include <__format/formatter.h>
 #include <__format/parser_std_format_spec.h>
+#include <__utility/unreachable.h>
 #include <array>
 #include <charconv>
 #include <concepts>
@@ -176,7 +177,7 @@ __determine_grouping(ptrdiff_t __size, const string& __grouping) {
     }
   }
 
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 template <class _Parser>
@@ -292,7 +293,7 @@ private:
     }
     default:
       _LIBCPP_ASSERT(false, "The parser should have validated the type");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
   }
 

libcxx/include/__iterator/advance.h

@@ -16,6 +16,7 @@
 #include <__iterator/incrementable_traits.h>
 #include <__iterator/iterator_traits.h>
 #include <__utility/move.h>
+#include <__utility/unreachable.h>
 #include <concepts>
 #include <cstdlib>
 #include <limits>
@@ -181,7 +182,7 @@ public:
       return __n;
     }
 
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 };
 

libcxx/include/__utility/unreachable.h

@@ -0,0 +1,38 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___UTILITY_UNREACHABLE_H
+#define _LIBCPP___UTILITY_UNREACHABLE_H
+
+#include <__config>
+#include <cstdlib>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI inline void __libcpp_unreachable()
+{
+#if __has_builtin(__builtin_unreachable)
+  __builtin_unreachable();
+#else
+  std::abort();
+#endif
+}
+
+#if _LIBCPP_STD_VER > 20
+
+[[noreturn]] _LIBCPP_HIDE_FROM_ABI inline void unreachable() { __libcpp_unreachable(); }
+
+#endif // _LIBCPP_STD_VER > 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif

libcxx/include/array

@@ -111,8 +111,8 @@ template <size_t I, class T, size_t N> const T&& get(const array<T, N>&&) noexce
 #include <__config>
 #include <__debug>
 #include <__tuple>
+#include <__utility/unreachable.h>
 #include <algorithm>
-#include <cstdlib> // for _LIBCPP_UNREACHABLE
 #include <iterator>
 #include <stdexcept>
 #include <type_traits>
@@ -309,54 +309,54 @@ struct _LIBCPP_TEMPLATE_VIS array<_Tp, 0>
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference operator[](size_type) _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::operator[] on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     const_reference operator[](size_type) const _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::operator[] on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference at(size_type) {
       __throw_out_of_range("array<T, 0>::at");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     const_reference at(size_type) const {
       __throw_out_of_range("array<T, 0>::at");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference front() _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::front() on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     const_reference front() const _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::front() on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference back() _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::back() on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     const_reference back() const _NOEXCEPT {
       _LIBCPP_ASSERT(false, "cannot call array<T, 0>::back() on a zero-sized array");
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
 };
 
 
-#if _LIBCPP_STD_VER >= 17
+#if _LIBCPP_STD_VER > 14
 template<class _Tp, class... _Args,
          class = enable_if_t<__all<_IsSame<_Tp, _Args>::value...>::value>
          >

libcxx/include/charconv

@@ -83,16 +83,16 @@ namespace std {
 #include <__charconv/from_chars_result.h>
 #include <__charconv/to_chars_result.h>
 #include <__config>
+#include <__debug>
 #include <__errc>
+#include <__utility/unreachable.h>
 #include <cmath> // for log2f
 #include <cstdint>
-#include <cstdlib> // for _LIBCPP_UNREACHABLE
+#include <cstdlib>
 #include <cstring>
 #include <limits>
 #include <type_traits>
 
-#include <__debug>
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
@@ -341,7 +341,7 @@ _LIBCPP_AVAILABILITY_TO_CHARS _LIBCPP_INLINE_VISIBILITY int __to_chars_integral_
     __r += 4;
   }
 
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 template <typename _Tp>

libcxx/include/cstdlib

@@ -88,12 +88,6 @@ void *aligned_alloc(size_t alignment, size_t size);                       // C11
 #  pragma GCC system_header
 #endif
 
-#ifdef __GNUC__
-#define _LIBCPP_UNREACHABLE() __builtin_unreachable()
-#else
-#define _LIBCPP_UNREACHABLE() _VSTD::abort()
-#endif
-
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 using ::size_t _LIBCPP_USING_IF_EXISTS;

libcxx/include/fstream

@@ -183,6 +183,7 @@ typedef basic_fstream<wchar_t> wfstream;
 #include <__config>
 #include <__debug>
 #include <__locale>
+#include <__utility/unreachable.h>
 #include <cstdio>
 #include <cstdlib>
 #include <istream>
@@ -538,7 +539,7 @@ const char* basic_filebuf<_CharT, _Traits>::__make_mdstring(
   default:
     return nullptr;
   }
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 template <class _CharT, class _Traits>

libcxx/include/module.modulemap

@@ -961,6 +961,7 @@ module std [system] {
       module swap                { private header "__utility/swap.h" }
       module to_underlying       { private header "__utility/to_underlying.h" }
       module transaction         { private header "__utility/transaction.h" }
+      module unreachable         { private header "__utility/unreachable.h" }
     }
   }
   module valarray {

libcxx/include/utility

@@ -239,6 +239,7 @@ template <class T>
 #include <__utility/swap.h>
 #include <__utility/to_underlying.h>
 #include <__utility/transaction.h>
+#include <__utility/unreachable.h>
 #include <compare>
 #include <initializer_list>
 #include <version>

libcxx/include/version

@@ -400,7 +400,7 @@ __cpp_lib_void_t                                        201411L <type_traits>
 # define __cpp_lib_string_contains                      202011L
 # define __cpp_lib_string_resize_and_overwrite          202110L
 # define __cpp_lib_to_underlying                        202102L
-// # define __cpp_lib_unreachable                          202202L
+# define __cpp_lib_unreachable                          202202L
 #endif
 
 // clang-format on

libcxx/src/filesystem/filesystem_common.h

@@ -14,11 +14,11 @@
 #include "chrono"
 #include "climits"
 #include "cstdarg"
-#include "cstdlib"
 #include "ctime"
 #include "filesystem"
 #include "ratio"
 #include "system_error"
+#include <utility>
 
 #if defined(_LIBCPP_WIN32API)
 # define WIN32_LEAN_AND_MEAN
@@ -178,7 +178,7 @@ struct ErrorHandler {
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 0)
@@ -197,7 +197,7 @@ struct ErrorHandler {
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 4)

libcxx/src/filesystem/operations.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__utility/unreachable.h>
 #include "filesystem"
 #include "array"
 #include "iterator"
@@ -154,7 +155,7 @@ public:
       return makeState(PS_AtEnd);
 
     case PS_AtEnd:
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
   }
 
@@ -202,7 +203,7 @@ public:
       return makeState(PS_InRootName, Path.data(), RStart + 1);
     case PS_InRootName:
     case PS_BeforeBegin:
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
   }
 
@@ -224,7 +225,7 @@ public:
     case PS_InFilenames:
       return RawEntry;
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   explicit operator bool() const noexcept {
@@ -285,7 +286,7 @@ private:
     case PS_AtEnd:
       return getAfterBack();
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   /// \brief Return a pointer to the first character in the currently lexed
@@ -302,7 +303,7 @@ private:
     case PS_AtEnd:
       return &Path.back() + 1;
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }
 
   // Consume all consecutive separators.
@@ -681,7 +682,7 @@ void filesystem_error::__create_what(int __num_paths) {
       return detail::format_string("filesystem error: %s [" PATH_CSTR_FMT "] [" PATH_CSTR_FMT "]",
                                    derived_what, path1().c_str(), path2().c_str());
     }
-    _LIBCPP_UNREACHABLE();
+    __libcpp_unreachable();
   }();
 }
 
@@ -1188,7 +1189,7 @@ bool __fs_is_empty(const path& p, error_code* ec) {
   } else if (is_regular_file(st))
     return static_cast<uintmax_t>(pst.st_size) == 0;
 
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 static file_time_type __extract_last_write_time(const path& p, const StatT& st,
@@ -1801,7 +1802,7 @@ path path::lexically_normal() const {
       break;
     }
     case PK_None:
-      _LIBCPP_UNREACHABLE();
+      __libcpp_unreachable();
     }
   }
   // [fs.path.generic]p6.8: If the path is empty, add a dot.

libcxx/src/locale.cpp

@@ -12,6 +12,7 @@
 #define _LCONV_C99
 #endif
 
+#include <__utility/unreachable.h>
 #include "algorithm"
 #include "clocale"
 #include "codecvt"
@@ -4623,7 +4624,7 @@ static bool checked_string_to_char_convert(char& dest,
 
   return false;
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-  _LIBCPP_UNREACHABLE();
+  __libcpp_unreachable();
 }
 
 

libcxx/src/strstream.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__utility/unreachable.h>
 #include "strstream"
 #include "algorithm"
 #include "climits"
@@ -268,7 +269,7 @@ strstreambuf::seekoff(off_type __off, ios_base::seekdir __way, ios_base::openmod
             newoff = seekhigh - eback();
             break;
         default:
-            _LIBCPP_UNREACHABLE();
+            __libcpp_unreachable();
         }
         newoff += __off;
         if (0 <= newoff && newoff <= seekhigh - eback())

libcxx/test/libcxx/diagnostics/detail.headers/utility/unreachable.module.verify.cpp

@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// 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: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__utility/unreachable.h'}}
+#include <__utility/unreachable.h>

libcxx/test/std/language.support/support.limits/support.limits.general/utility.version.pass.cpp

@@ -297,17 +297,11 @@
 #   error "__cpp_lib_tuples_by_type should have the value 201304L in c++2b"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_unreachable
-#     error "__cpp_lib_unreachable should be defined in c++2b"
-#   endif
-#   if __cpp_lib_unreachable != 202202L
-#     error "__cpp_lib_unreachable should have the value 202202L in c++2b"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_unreachable
-#     error "__cpp_lib_unreachable should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_unreachable
+#   error "__cpp_lib_unreachable should be defined in c++2b"
+# endif
+# if __cpp_lib_unreachable != 202202L
+#   error "__cpp_lib_unreachable should have the value 202202L in c++2b"
 # endif
 
 #endif // TEST_STD_VER > 20

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.pass.cpp

@@ -5005,17 +5005,11 @@
 #   error "__cpp_lib_unordered_map_try_emplace should have the value 201411L in c++2b"
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_unreachable
-#     error "__cpp_lib_unreachable should be defined in c++2b"
-#   endif
-#   if __cpp_lib_unreachable != 202202L
-#     error "__cpp_lib_unreachable should have the value 202202L in c++2b"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_unreachable
-#     error "__cpp_lib_unreachable should not be defined because it is unimplemented in libc++!"
-#   endif
+# ifndef __cpp_lib_unreachable
+#   error "__cpp_lib_unreachable should be defined in c++2b"
+# endif
+# if __cpp_lib_unreachable != 202202L
+#   error "__cpp_lib_unreachable should have the value 202202L in c++2b"
 # endif
 
 # ifndef __cpp_lib_unwrap_ref

libcxx/test/std/utilities/utility/utility.unreachable/unreachable.compile.pass.cpp

@@ -0,0 +1,14 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+#include <utility>
+#include <type_traits>
+
+static_assert(std::is_same_v<decltype(std::unreachable()), void>);

libcxx/test/std/utilities/utility/utility.unreachable/unreachable.verify.cpp

@@ -0,0 +1,13 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+#include <utility>
+
+[[noreturn]] void unreachable() { std::unreachable(); } // expected-no-diagnostics

libcxx/utils/generate_feature_test_macro_components.py

@@ -744,7 +744,6 @@ feature_test_macros = [ add_version_header(x) for x in [
     "name": "__cpp_lib_unreachable",
     "values": { "c++2b": 202202 },
     "headers": ["utility"],
-    "unimplemented": True,
   }, {
     "name": "__cpp_lib_unwrap_ref",
     "values": { "c++20": 201811 },