Why must we do this:

// concept C
template <C ...T >
decltype(auto) func (T ...args)
{
          return args + ...;
}

So unnecessary!

Why not:

decltype(auto) func (C ...args)
{
          Return args + ...;
}

??

Not hard to implement, as auto in param list is already implemented.
This would help lots, & would speed up coding, making it more maintainable too!