Specify concepts for all template arguments

Reason

Correctness and readability. The assumed meaning (syntax and semantics) of a template argument is fundamental to the interface of a template. A concept dramatically improves documentation and error handling for the template. Specifying concepts for template arguments is a powerful design tool.

Example

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

or equivalently and more succinctly:

template<input_iterator Iter, typename Val>
    requires equality_comparable_with<iter_value_t<Iter>, Val>
Iter find(Iter b, Iter e, Val v)
{
    // ...
}

Note

Plain typename (or auto) is the least constraining concept. It should be used only rarely when nothing more than "it's a type" can be assumed. This is typically only needed when (as part of template metaprogramming code) we manipulate pure expression trees, postponing type checking.

References: TC++PL4

Enforcement

Flag template type arguments without concepts