Advanced SFINAE

Next, we consider a problem that is simply stated, although not as simply solved. We are writing generic code that operates on a container object of an arbitrary T type. At some point, we need to sort the data in this container. We assume that, if the container provides a T::sort() member function, then this is the best way to sort the data (the implementer of the container presumably knows how the data is organized). If there is no such member function, but the container is a sequence with the begin() and end() member functions, then we can call std::sort() on that sequence. Otherwise, we don't know how to sort the data and our code should not compile.

A naive attempt to solve the problem might look like this:

template <typename ...

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