On Wed, Mar 5, 2025 at 2:57 PM Ville Voutilainen
<ville.voutilainen_at_[hidden]> wrote:
>
>
> https://wg21.link/p3096
>
> is the paper you're looking for. It's been approved by EWG and should
> progress into spec reviews.


Then the constructor of 'std::function_defargs' would look as follows:

        template<typename FuncPtrParam, typename... Ts, size_t... I>
        consteval void constructor_proper(FuncPtrParam fptr,
tuple<Ts...> &&tup, index_sequence<I...>)
        {
            constexpr auto mylambda =
                []<size_t I2, typename Tuple>(Tuple &&tup2,
function_defargs<T> *const this2, FuncPtrParam fptr2)
                {
                    constexpr auto params = meta::parameters_of( ^^fptr2 );
                    static_assert( params.size() == tuple_size_v<Tuple> );
                    if ( false ==
meta::has_default_argument(params[I2]) ) return;
                    using ArgType = tuple_element_t<I2,Tuple>;
                    get<I2>(this2->func_args) =
meta::get_default_argument( params[I2] );
                };

            ( mylambda.template operator()<I>( move(tup), this, fptr ), ... );
        }

The only problem though here is that there's no such thing as
"meta::get_default_argument". That paper has a feature for detecting
the presence of default arguments but it doesn't provide a means of
getting the value of the default argument.