What is defined behavior of std::upper_bound on an empty end range?

Question

I'm trying to understand in C++ whether the standard allows as valid a call to std::upper_bound with a range that is the end() of a container to its end()?

In other words, is the empty range [end(), end()) a valid range? Is end() reachable from itself? Does the answer remain the same in C++98 as it does in C++11?

Answer 1

[first, last)

Standard says that [first, last) is not a valid range if first is behind the last in the range. That makes [cont.end(), cont.end()] a valid range. However, algorithms are no-op for these ranges. For e.g:-

std::reverse(cont.end(), cont.end());

is just a no-op rather than undefined behavior.