[ncmncm]

The most important thing to say about this "definitive guide" is that it delays to the end presenting the overwhelmingly most important detail about Concepts: how to use them.

The right place to put a concept name, in production code, is in place of "typename" in a template definition, or even better in place of "T" in the function argument declaration. That is, instead of

  template<typename T>
  T add(T a, T b)
      requires addable<T> (
    return a + b;
  )

say

  template <addable T>
  T add(T a, T b) {
    return a + b;
  }

or

  auto add(
      addable auto a, 
      addable auto b) {
    return a + b;
  }

according as whether you want to enforce a and b to have the same type (which is another omission).

Most often it is not necessary, and not wanted, to enforce a and b having the same type.

[CodeMage]

> Most often it is not necessary, and not wanted, to enforce a and b having the same type.

That really depends on what you're trying to do. Presenting these two different declarations as somehow equivalent is very misleading and I'm glad that the author didn't do that.

[ncmncm]

They are not presented as equivalent, but as a choice.

The author's failing was in presenting neither of them until long after they were due, and presenting thoroughly inferior alternatives in the meantime.