Does this conflict with the specification of 'std::ranges::to'? From my (naive) reading, the 'DEDUCE_EXPR' for when a template, such as 'std::vector', is passed to 'std::ranges::to' relies on being able to use the template template parameter 'C' as a deducible template. Or is the standard just given special blessing to have it work as if 'C' were a deducible template?

Wrote some code the other day that mimicked the 'DEDUCE_EXPR' for 'std::ranges::to', so it'd be nice to know if I should go change that now.


On Monday, September 22nd, 2025 at 1:17 PM, Jens Maurer via Std-Discussion <std-discussion_at_[hidden]> wrote:

>
> On 9/22/25 09:11, Ell via Std-Discussion wrote:
>
> > Consider this:
> >
> > template <class T, class U = long>
> > struct A { A (T, U); };
> >
> > template <class T>
> > using B = A<T>;
> >
> > template <template <class> class P>
> > P x (1, 2);
> >
> > The type of x<B> is A<int, long>, as expected. However, the type of X<A>
> > is A<int, int>, which is surprising. I'd expect it to either
> > be A<int, long>, as if P were an alias template, or to be ill formed,
> > since the deduced type doesn't match P.
>
>
> This is ill-formed; the name of a template template parameter is not
> a deducible template per the current rules, as clarified here:
>
> https://cplusplus.github.io/CWG/issues/3003.html
>
> If you wish to make this situation deducible, please submit a paper
> to EWG.
>
> Jens
>
> --
> Std-Discussion mailing list
> Std-Discussion_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-discussion