On 9/23/25 01:48, Keenan Horrigan wrote:
> 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?

I've posted a library issue and amended CWG3003 accordingly.

Good catch!

Jens


> 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
>>
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