lst-0068-book.cpp
// https://godbolt.org/z/9qsGW6M8Y
template<std::ranges::range T> // a range over type T
using ValueTypeOfRange = std::ranges::range_value_t<T>;
// or
template<typename T>
requires std::ranges::range<T> // a range over type T
using ValueTypeOfRange = std::ranges::range_value_t<T>;
// results in:
ValueTypeOfRange<std::vector<int>> x; // x is int
ValueTypeOfRange<std::string> y; // y is char
ValueTypeOfRange<std::list<double>> z; // z is double
ValueTypeOfRange<int> w; // (ERR) int is not a range