Merge 2022-02 LWG Motion 4

P0627R6 Function to mark unreachable code

Author: tkoeppe

source/support.tex

@@ -707,6 +707,7 @@
 #define @\defnlibxname{cpp_lib_type_trait_variable_templates}@     201510L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_uncaught_exceptions}@               201411L // also in \libheader{exception}
 #define @\defnlibxname{cpp_lib_unordered_map_try_emplace}@         201411L // also in \libheader{unordered_map}
+#define @\defnlibxname{cpp_lib_unreachable}@                       202202L // also in \libheader{utility}
 #define @\defnlibxname{cpp_lib_unwrap_ref}@                        201811L // also in \libheader{type_traits}
 #define @\defnlibxname{cpp_lib_variant}@                           202106L // also in \libheader{variant}
 #define @\defnlibxname{cpp_lib_void_t}@                            201411L // also in \libheader{type_traits}

source/utilities.tex

@@ -99,6 +99,9 @@
   template<class T>
     constexpr underlying_type_t<T> to_underlying(T value) noexcept;
 
+  // \ref{utility.unreachable}, unreachable
+  [[noreturn]] void unreachable();
+
   // \ref{intseq}, compile-time integer sequences%
 \indexlibraryglobal{index_sequence}%
 \indexlibraryglobal{make_index_sequence}%
@@ -611,6 +614,39 @@
 \tcode{static_cast<underlying_type_t<T>>(value)}.
 \end{itemdescr}
 
+\rSec2[utility.unreachable]{Function \tcode{unreachable}}
+
+\indexlibraryglobal{unreachable}%
+\begin{itemdecl}
+[[noreturn]] void unreachable();
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\expects
+\tcode{false} is \tcode{true}.
+\begin{note}
+This precondition cannot be satisfied, thus the behavior
+of calling \tcode{unreachable} is undefined.
+\end{note}
+
+\pnum
+\begin{example}
+\begin{codeblock}
+int f(int x) {
+  switch (x) {
+  case 0:
+  case 1:
+    return x;
+  default:
+    std::unreachable();
+  }
+}
+int a = f(1);           // OK, \tcode{a} has value \tcode{1}
+int b = f(3);           // undefined behavior
+\end{codeblock}
+\end{example}
+\end{itemdescr}
 
 \rSec1[intseq]{Compile-time integer sequences}