Putting it through a for loop demonstrates that it is iterable, imo. template <typename T> void foo(const T& container) { for (auto value : container)

Putting it through a for loop demonstrates that it is iterable, imo. template <typename T> void foo(const T& container) { for (auto value : container) { // process value } } foo(std::vector<int>()) foo(std::optional<int>())