[PATCH v2 2/2] libstdc++: Make ref_view<R> statically sized if R is statically sized

[Hewill Kang]

>
> you should also require that integer_class types are structural

I have already submitted an LWG for this today.

  I think consteval ranges::static_size<Rg>() function would be better that
> variable, this is not something
>   we want to emit in binary (because someone took reference to it)

 range_static_size_v basically follows the naming convention of the
currently available range_size_t.

inplace_vector could also use mandates on append_range, you cannot append
> more than capacity

 I don't know how this can be mandates, because the current size of the
inplace_vector itself is not a constant expression.

Add some discussion regarding the breaks (not a Precondition to Mandate)
> that the paper is adding:

Yes, that will be more complete, thank you.

Tomasz Kaminski <tkaminsk@redhat.com> 於 2026年3月19日週四 下午3:18寫道：

>
>
> On Wed, Mar 18, 2026 at 5:55 PM Hewill Kang <hewillk@gmail.com> wrote:
>
>> I wrote a similar paper, namely
>> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3928r0.html
>> If you want to enhance ref_view::size, then I think ref_view::empty
>> should also be enhanced, just like the paper did.
>>
> Thanks, here are some suggestions:
> * you should also require that integer_class types are structural
> * I think consteval ranges::static_size<Rg>() function would be better
> that variable, this is not something
>   we want to emit in binary (because someone took reference to it)
> * inplace_vector could also use mandates on append_range, you cannot
> append more than capacity
>
> Add some discussion regarding the breaks (not a Precondition to Mandate)
> that the paper is adding:
> * span CTAD deduces sized range
> * join_view, join_with_view is now sized sometimes
>  * split_view
>
>>
>> Tomasz Kamiński <tkaminsk@redhat.com> 於 2026年3月19日週四 上午12:31寫道：
>>
>>> If the referenced range is statically sized, instead of referencing a
>>> pointer whose value is not know at compile time, we return
>>> integral_constant<range_difference_t<_Rg>, ranges::size(__rg)>::value.
>>>
>>> libstdc++-v3/ChangeLog:
>>>
>>>         * include/std/ranges (ref_view::size()): Return constant for
>>> statically
>>>         sized ranges, even if pointed object is unknown.
>>>         * testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
>>>         Expect errors from ref_view of array examples.
>>> ---
>>> v2 uses integral_constant to produce a value of ranges size.
>>>
>>>  libstdc++-v3/include/std/ranges                | 18 +++++++++++++++++-
>>>  .../inplace_vector/cons/from_range_neg.cc      |  6 ++----
>>>  2 files changed, 19 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/libstdc++-v3/include/std/ranges
>>> b/libstdc++-v3/include/std/ranges
>>> index 95e0109a10c..71ab0d674c9 100644
>>> --- a/libstdc++-v3/include/std/ranges
>>> +++ b/libstdc++-v3/include/std/ranges
>>> @@ -1395,7 +1395,23 @@ namespace views::__adaptor
>>>
>>>        constexpr auto
>>>        size() const requires sized_range<_Range>
>>> -      { return ranges::size(*_M_r); }
>>> +      {
>>> +#if __cplusplus > 202302L
>>> +       if constexpr (__static_sized_range<_Range>)
>>> +         {
>>> +           auto __conjure_size_constant
>>> +             = [](_Range& __rg)
>>> +               {
>>> +                 return integral_constant<
>>> +                          range_difference_t<_Range>,
>>> +                          ranges::size(__rg)>();
>>> +               };
>>> +            return decltype(__conjure_size_constant(*_M_r))::value;
>>> +         }
>>> +        else
>>> +#endif
>>> +         return ranges::size(*_M_r);
>>> +      }
>>>
>>>        constexpr auto
>>>        data() const requires contiguous_range<_Range>
>>> diff --git
>>> a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>>> b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>>> index cf87129da51..15fd7b5c285 100644
>>> ---
>>> a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>>> +++
>>> b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>>> @@ -57,14 +57,12 @@ test_all()
>>>    test_one(a1); // { dg-error "from here" }
>>>    test_one(s1); // { dg-error "from here" }
>>>    test_one(s5 | std::views::adjacent<5> | std::views::elements<0>); //
>>> { dg-error "from here" }
>>> -  // ref_view is not statically sized due pointer dereference
>>> -  test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
>>> +  test_one(a5 | std::views::adjacent<5> | std::views::elements<0>); //
>>> { dg-error "from here" }
>>>
>>>    test_five(a5); // { dg-error "from here" }
>>>    test_five(s5); // { dg-error "from here" }
>>>    test_five(s7 | std::views::adjacent<3> | std::views::elements<0>); //
>>> { dg-error "from here" }
>>> -  // ref_view is not statically sized due pointer dereference
>>> -  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
>>> +  test_five(a7 | std::views::adjacent<3> | std::views::elements<0>); //
>>> { dg-error "from here" }
>>>  }
>>>
>>>  // { dg-error "static assertion failed" "" { target *-*-* } 0 }
>>> --
>>> 2.53.0
>>>
>>>
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