message: Add `match_const_t` alias.
diff: @@ -45,6 +45,9 @@ using match_const =
  typename std::conditional<std::is_const<Other>::value, std::add_const<T>,
  std::remove_const<T>>::type;
  +template <class T, class Other>
  +using match_const_t = typename match_const<T, Other>::type;
  +
  // Detection idiom from
  // C++ Extensions for Library Fundamentals, Version 2 (ISO/IEC TS 19568:2017)
  // https://en.cppreference.com/w/cpp/experimental/is_detected
  