message: Add no-trait base OpImpl::Base.
diff: @@ -116,8 +116,11 @@ protected:
  : state(const_cast<Operation *>(state)) {}
  private:
  + template <typename... Types>
  + struct BaseVerifier;
  +
  template <typename First, typename... Rest>
  - struct BaseVerifier {
  + struct BaseVerifier<First, Rest...> {
  static const char *verifyBase(const Operation *op) {
  if (auto error = First::verifyBase(op))
  return error;
  @@ -132,6 +135,13 @@ private:
  }
  };
  + template <>
  + struct BaseVerifier<> {
  + static const char *verifyBase(const Operation *op) {
  + return nullptr;
  + }
  + };
  +
  Operation *state;
  };
  