Merge 2025-02 LWG Motion 4

P0472R3 Put std::monostate in <utility>

Author: tkoeppe

source/utilities.tex

@@ -221,6 +221,21 @@
       explicit nontype_t() = default;
     };
   template<auto V> constexpr nontype_t<V> nontype{};
+
+  // \ref{variant.monostate}, class \tcode{monostate}%
+\indexlibraryglobal{monostate}
+  struct monostate;
+
+  // \ref{variant.monostate.relops}, \tcode{monostate} relational operators%
+\indexlibrarymember{operator==}{monostate}%
+\indexlibrarymember{operator<=>}{monostate}
+  constexpr bool operator==(monostate, monostate) noexcept;
+  constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
+
+  // \ref{variant.hash}, hash support%
+\indexlibrarymember{hash}{monostate}
+  template<class T> struct hash;
+  template<> struct hash<monostate>;
 }
 \end{codeblock}