[tem.param] Template heads of concept definitions

[cor3ntin]

If a template-parameter of a class template, variable template, or alias template has a default template argument, each subsequent template-parameter shall either have a default template argument supplied or declare a template parameter pack. If a template-parameter of a primary class template, primary variable template, or alias template declares a template parameter pack, it shall be the last template-parameter.

Per this logic, all of these are valid concept definitions.

template <typename T, typename U = int, typename V>
concept C = true;

template <typename T = int>
concept C2 = true;
static_assert(C2<>);

template <typename... T, typename... U>
concept C3 = true;


template <typename... T, typename U = int>
concept C4 = true;


template <typename... T, typename U>
concept C5 = true;

There is wild implementation divergence https://compiler-explorer.com/z/z376fMYYr

Additionally, the C2 case is interesting.

in [concept.definition] we say:

The first declared template parameter of a concept definition is its prototype parameter

But can that parameter be defaulted?

I assume we want none of these examples to be valid.

Suggested resolution

Modify the last paragraph of [temp.concept]

The first declared template parameter of a concept definition is its prototype parameter. It shall not have a default argument [temp.param]. A type concept is a concept whose prototype parameter is a type template parameter.

Modify [temp.param]/p19

If a template-parameter of a class template, variable template, concept, or alias template has a default template argument, each subsequent template-parameter shall either have a default template argument supplied or declare a template parameter pack. If a template-parameter of a primary class template, primary variable template, concept, or alias template declares a template parameter pack, it shall be the last template-parameter.