Currently std::ignore is not specified constexpr according to the C++ draft N4606 in the paragraph [tuple.general]. It prevents some use in constexpr context: for example declaring a constexpr variable equals to the result of a function to which std::ignore has been passed as a parameter:

constexpr int i = f(std::ignore); // Won't compile

If there is no fundamental reason preventing std::ignore to be constexpr, then we propose to declare it as constexpr instead of as const.