Differences between std::is_integer and std::is_integral?

std::is_integer<T> does not exist.

That being said, std::numeric_limits<T>::is_integer does exist.

I’m not aware of any significant difference between std::numeric_limits<T>::is_integer and std::is_integral<T>. The latter was designed much later and became standard in C++11, whereas the former was introduced in C++98.