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

[Tomasz Kaminski]

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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