Merge pull request hsutter#49 from filipsajdak/fsajdak-string-interpolation-composition-of-standard-types

[FIX] Add handling of composition of standard types

Author: hsutter

include/cpp2util.h

@@ -771,6 +771,15 @@ inline auto to_string(T const& sv) -> std::string
  return std::string{sv};
 }
 
+template <typename... Ts>
+inline auto to_string(std::variant<Ts...> const& v) -> std::string;
+
+template < typename T, typename U>
+inline auto to_string(std::pair<T,U> const& p) -> std::string;
+
+template < typename... Ts>
+inline auto to_string(std::tuple<Ts...> const& t) -> std::string;
+
 template<typename T>
 inline auto to_string(std::optional<T> const& o) -> std::string {
  if (o.has_value()) {