diff --git a/libcxx/docs/Status/Cxx2bPapers.csv b/libcxx/docs/Status/Cxx2bPapers.csv
--- a/libcxx/docs/Status/Cxx2bPapers.csv
+++ b/libcxx/docs/Status/Cxx2bPapers.csv
@@ -38,3 +38,5 @@
 "`P2393R1 <https://wg21.link/P2393R1>`__","LWG","Cleaning up ``integer``-class types","October 2021","",""
 "`P2401R0 <https://wg21.link/P2401R0>`__","LWG","Add a conditional ``noexcept`` specification to ``std::exchange``","October 2021","|Complete|","14.0"
 "","","","","",""
+"`P0627R6 <https://wg21.link/P0627R6>`__","LWG","Function to mark unreachable code","February 2022","|Complete|"","15.0"
+"","","","","",""
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -397,6 +397,7 @@
   __utility/swap.h
   __utility/to_underlying.h
   __utility/transaction.h
+  __utility/unreachable.h
   __variant/monostate.h
   algorithm
   any
diff --git a/libcxx/include/__filesystem/directory_entry.h b/libcxx/include/__filesystem/directory_entry.h
--- a/libcxx/include/__filesystem/directory_entry.h
+++ b/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 @@
         __ec->clear();
       return __data_.__type_;
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -383,7 +384,7 @@
       return __data_.__type_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -398,7 +399,7 @@
     case _RefreshSymlink:
       return file_status(__get_ft(__ec), __data_.__non_sym_perms_);
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -414,7 +415,7 @@
     case _RefreshSymlinkUnresolved:
       return file_status(__get_sym_ft(__ec), __data_.__sym_perms_);
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -439,7 +440,7 @@
       return __data_.__size_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -458,7 +459,7 @@
       return __data_.__nlink_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_INLINE_VISIBILITY
@@ -481,7 +482,7 @@
       return __data_.__write_time_;
     }
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
 private:
diff --git a/libcxx/include/__format/format_arg.h b/libcxx/include/__format/format_arg.h
--- a/libcxx/include/__format/format_arg.h
+++ b/libcxx/include/__format/format_arg.h
@@ -17,6 +17,7 @@
 #include <__format/format_parse_context.h>
 #include <__functional_base>
 #include <__memory/addressof.h>
+#include <__utility/unreachable.h>
 #include <__variant/monostate.h>
 #include <string>
 #include <string_view>
@@ -81,7 +82,7 @@
 #ifndef _LIBCPP_HAS_NO_INT128
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__i128);
 #else
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
 #endif
   case __format::__arg_t::__unsigned:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__unsigned);
@@ -92,7 +93,7 @@
 #ifndef _LIBCPP_HAS_NO_INT128
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__u128);
 #else
-   _LIBCPP_UNREACHABLE();
+   __unreachable();
 #endif
   case __format::__arg_t::__float:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__float);
@@ -110,7 +111,7 @@
   case __format::__arg_t::__handle:
     return _VSTD::invoke(_VSTD::forward<_Visitor>(__vis), __arg.__handle);
   }
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 template <class _Context>
diff --git a/libcxx/include/__format/formatter.h b/libcxx/include/__format/formatter.h
--- a/libcxx/include/__format/formatter.h
+++ b/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 <concepts>
 #include <string_view>
 
@@ -120,7 +121,7 @@
   size_t __fill = __width - __size;
   switch (__align) {
   case __format_spec::_Flags::_Alignment::__default:
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
 
   case __format_spec::_Flags::_Alignment::__left:
     return {0, __fill};
@@ -136,7 +137,7 @@
   case __format_spec::_Flags::_Alignment::__right:
     return {__fill, 0};
   }
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 /**
diff --git a/libcxx/include/__format/formatter_floating_point.h b/libcxx/include/__format/formatter_floating_point.h
--- a/libcxx/include/__format/formatter_floating_point.h
+++ b/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 @@
 
     default:
       _LIBCPP_ASSERT(false, "The parser should have validated the type");
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
   }
 };
diff --git a/libcxx/include/__format/formatter_integral.h b/libcxx/include/__format/formatter_integral.h
--- a/libcxx/include/__format/formatter_integral.h
+++ b/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 @@
     }
   }
 
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 template <class _Parser>
@@ -292,7 +293,7 @@
     }
     default:
       _LIBCPP_ASSERT(false, "The parser should have validated the type");
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
   }
 
diff --git a/libcxx/include/__iterator/advance.h b/libcxx/include/__iterator/advance.h
--- a/libcxx/include/__iterator/advance.h
+++ b/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 @@
       return __n;
     }
 
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 };
 
diff --git a/libcxx/include/__utility/unreachable.h b/libcxx/include/__utility/unreachable.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__utility/unreachable.h
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+// 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>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_NORETURN _LIBCPP_HIDE_FROM_ABI inline void __unreachable()
+{
+#ifdef __GNUC__
+  __builtin_unreachable();
+#else
+  std::abort();
+#endif
+}
+
+#if _LIBCPP_STD_VER > 20
+
+[[noreturn]] _LIBCPP_HIDE_FROM_ABI inline void unreachable() { __unreachable(); }
+
+#endif // _LIBCPP_STD_VER > 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif
diff --git a/libcxx/include/array b/libcxx/include/array
--- a/libcxx/include/array
+++ b/libcxx/include/array
@@ -111,8 +111,8 @@
 #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 @@
     _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();
+      __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();
+      __unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
     reference at(size_type) {
       __throw_out_of_range("array<T, 0>::at");
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     const_reference at(size_type) const {
       __throw_out_of_range("array<T, 0>::at");
-      _LIBCPP_UNREACHABLE();
+      __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();
+      __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();
+      __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();
+      __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();
+      __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>
          >
diff --git a/libcxx/include/charconv b/libcxx/include/charconv
--- a/libcxx/include/charconv
+++ b/libcxx/include/charconv
@@ -84,6 +84,7 @@
 #include <__charconv/to_chars_result.h>
 #include <__config>
 #include <__errc>
+#include <__utility/unreachable.h>
 #include <cmath> // for log2f
 #include <cstdint>
 #include <cstdlib> // for _LIBCPP_UNREACHABLE
@@ -341,7 +342,7 @@
     __r += 4;
   }
 
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 template <typename _Tp>
diff --git a/libcxx/include/cstdlib b/libcxx/include/cstdlib
--- a/libcxx/include/cstdlib
+++ b/libcxx/include/cstdlib
@@ -88,12 +88,6 @@
 #  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;
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -183,6 +183,7 @@
 #include <__config>
 #include <__debug>
 #include <__locale>
+#include <__utility/unreachable.h>
 #include <cstdio>
 #include <cstdlib>
 #include <istream>
@@ -538,7 +539,7 @@
   default:
     return nullptr;
   }
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 template <class _CharT, class _Traits>
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -953,6 +953,7 @@
       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 {
diff --git a/libcxx/include/utility b/libcxx/include/utility
--- a/libcxx/include/utility
+++ b/libcxx/include/utility
@@ -239,6 +239,7 @@
 #include <__utility/swap.h>
 #include <__utility/to_underlying.h>
 #include <__utility/transaction.h>
+#include <__utility/unreachable.h>
 #include <compare>
 #include <initializer_list>
 #include <version>
diff --git a/libcxx/src/filesystem/filesystem_common.h b/libcxx/src/filesystem/filesystem_common.h
--- a/libcxx/src/filesystem/filesystem_common.h
+++ b/libcxx/src/filesystem/filesystem_common.h
@@ -10,6 +10,7 @@
 #define FILESYSTEM_COMMON_H
 
 #include "__config"
+#include <__utility/unreachable.h>
 #include "array"
 #include "chrono"
 #include "climits"
@@ -178,7 +179,7 @@
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 0)
@@ -197,7 +198,7 @@
     case 2:
       __throw_filesystem_error(what, *p1_, *p2_, ec);
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   _LIBCPP_ATTRIBUTE_FORMAT(__printf__, 3, 4)
diff --git a/libcxx/src/filesystem/operations.cpp b/libcxx/src/filesystem/operations.cpp
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__utility/unreachable.h>
 #include "filesystem"
 #include "array"
 #include "iterator"
@@ -154,7 +155,7 @@
       return makeState(PS_AtEnd);
 
     case PS_AtEnd:
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
   }
 
@@ -202,7 +203,7 @@
       return makeState(PS_InRootName, Path.data(), RStart + 1);
     case PS_InRootName:
     case PS_BeforeBegin:
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
   }
 
@@ -224,7 +225,7 @@
     case PS_InFilenames:
       return RawEntry;
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   explicit operator bool() const noexcept {
@@ -285,7 +286,7 @@
     case PS_AtEnd:
       return getAfterBack();
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   /// \brief Return a pointer to the first character in the currently lexed
@@ -302,7 +303,7 @@
     case PS_AtEnd:
       return &Path.back() + 1;
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }
 
   // Consume all consecutive separators.
@@ -681,7 +682,7 @@
       return detail::format_string("filesystem error: %s [" PATH_CSTR_FMT "] [" PATH_CSTR_FMT "]",
                                    derived_what, path1().c_str(), path2().c_str());
     }
-    _LIBCPP_UNREACHABLE();
+    __unreachable();
   }();
 }
 
@@ -1188,7 +1189,7 @@
   } else if (is_regular_file(st))
     return static_cast<uintmax_t>(pst.st_size) == 0;
 
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 static file_time_type __extract_last_write_time(const path& p, const StatT& st,
@@ -1801,7 +1802,7 @@
       break;
     }
     case PK_None:
-      _LIBCPP_UNREACHABLE();
+      __unreachable();
     }
   }
   // [fs.path.generic]p6.8: If the path is empty, add a dot.
diff --git a/libcxx/src/locale.cpp b/libcxx/src/locale.cpp
--- a/libcxx/src/locale.cpp
+++ b/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 @@
 
   return false;
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-  _LIBCPP_UNREACHABLE();
+  __unreachable();
 }
 
 
diff --git a/libcxx/src/strstream.cpp b/libcxx/src/strstream.cpp
--- a/libcxx/src/strstream.cpp
+++ b/libcxx/src/strstream.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__utility/unreachable.h>
 #include "strstream"
 #include "algorithm"
 #include "climits"
@@ -268,7 +269,7 @@
             newoff = seekhigh - eback();
             break;
         default:
-            _LIBCPP_UNREACHABLE();
+            __unreachable();
         }
         newoff += __off;
         if (0 <= newoff && newoff <= seekhigh - eback())