Use template metaprogramming primarily to emulate concepts

Reason

Where C++20 is not available, we need to emulate them using TMP. Use cases that require concepts (e.g. overloading based on concepts) are among the most common (and simple) uses of TMP.

Example

template<typename Iter>
    /*requires*/ enable_if<random_access_iterator<Iter>, void>
advance(Iter p, int n) { p += n; }

template<typename Iter>
    /*requires*/ enable_if<forward_iterator<Iter>, void>
advance(Iter p, int n) { assert(n >= 0); while (n--) ++p;}

Note

Such code is much simpler using concepts:

void advance(random_access_iterator auto p, int n) { p += n; }

void advance(forward_iterator auto p, int n) { assert(n >= 0); while (n--) ++p;}

Enforcement

???