P0472R3 Put std::monostate in <utility>

Author: eisenwave

source/utilities.tex

@@ -220,6 +220,17 @@
       explicit nontype_t() = default;
     };
   template<auto V> constexpr nontype_t<V> nontype{};
+
+  // \ref{variant.monostate}, class \tcode{monostate}
+  struct monostate;
+
+  // \ref{variant.monostate.relops}, \tcode{monostate} relational operators
+  constexpr bool operator==(monostate, monostate) noexcept;
+  constexpr strong_ordering operator<=>(monostate, monostate) noexcept;
+
+  // \ref{variant.hash}, hash support
+  template<class T> struct hash;
+  template<> struct hash<monostate>;
 }
 \end{codeblock}