Source: Add std::unreachable from C++23

Utilities/std/cm/utility

@@ -9,6 +9,13 @@
 #  define CMake_HAVE_CXX_IN_PLACE
 #endif
 
+#if (__cplusplus >= 202302L ||                                                \
+     (defined(_MSVC_LANG) && _MSVC_LANG >= 202302L)) &&                       \
+  defined(__cpp_lib_unreachable) && (__cpp_lib_unreachable >= 202202L)
+#  define CMake_HAVE_CXX_UNREACHABLE
+#endif
+
+#include <cstdlib> // IWYU pragma: export
 #include <utility> // IWYU pragma: export
 
 namespace cm {
@@ -27,5 +34,32 @@ struct in_place_t
 
 constexpr in_place_t in_place{};
 
+#endif
+
+#if defined(CMake_HAVE_CXX_UNREACHABLE)
+
+using std::unreachable;
+
+#else
+
+[[noreturn]] inline void unreachable()
+{
+#  if defined(_MSC_VER)
+  __assume(0);
+  std::abort();
+#  elif defined(__has_builtin)
+#    if __has_builtin(__builtin_unreachable)
+  __builtin_unreachable();
+#    else
+  std::abort();
+#    endif
+#  elif defined(__GNUC__) && defined(__GNUC_MINOR__) &&                       \
+    ((__GNUC__ * 100 + __GNUC_MINOR__) >= 405)
+  __builtin_unreachable();
+#  else
+  std::abort();
+#  endif
+}
+
 #endif
 }