message: utils.hh: work around MSVC STL bug
diff: @@ -132,7 +132,7 @@ public:
  template <typename T> struct Vec {
  std::unique_ptr<T[]> a;
  int s = 0;
  -#if !(__clang__ || __GNUC__ > 7 || __GNUC__ == 7 && __GNUC_MINOR__ >= 4)
  +#if !(__clang__ || __GNUC__ > 7 || __GNUC__ == 7 && __GNUC_MINOR__ >= 4) || defined(_WIN32)
  // Work around a bug in GCC<7.4 that optional<IndexUpdate> would not be
  // construtible.
  Vec() = default;
  