How to test if a type exists in a concept?

Question

How can I test if a member type exists in a concept template argument, i.e. for typename Container, test for Container::reverse_iterator ? What's the proper requires-clause ?

Answer 1

If you just want the type to exist, follow answer of @StoryTeller above.

If you want the type to exist and fulfill a specific concept, then do the:

template<class Container>
concept has_reverse_iterator
   = std::random_access_iterator<typename Container::reverse_iterator>;

Answer 2

We do this with the aptly named type requirement:

template<class Container>
concept has_reverse_iterator = requires {
    typename Container::reverse_iterator;
};