If an interface is a template, document its parameters using concepts

Reason

Make the interface precisely specified and compile-time checkable in the (not so distant) future.

Example

Use the C++20 style of requirements specification. For example:

template<typename Iter, typename Val>
  requires input_iterator<Iter> && equality_comparable_with<iter_value_t<Iter>, Val>
Iter find(Iter first, Iter last, Val v)
{
    // ...
}

See also: Generic programming and concepts.

Enforcement

Warn if any non-variadic template parameter is not constrained by a concept (in its declaration or mentioned in a requires clause).