///         contain a value of type `T`, the program terminates via a call to
  ///         `assert`.
  template <typename T>
  T& get() {
    assert(type_ == EnumValueOf<T>::value);
    return UnionAccess<T>::unsafe_reference(*this);
  }

  /// @brief Get the underlying value based on type.
  ///
  /// @tparam T The desired return type.