On Fri, 24 Apr 2026, Tomasz Kamiński wrote:
> This patch introduces ranges::__static_size<_Range> helper functions,
> that returns ranges::size(__rg) for __statically_sized_range.
> This function is then used for ref_view<R>::size if R has static size,
> avoiding derefence of pointer value that is not know at compile time.
> Similary for ref_view<R>::empty() we compare the size with zero,
> if it is know statically.
>
> This implements revelant part of P3928R0: static_sized_range by Hewill Kang.
> As standard does not specify when constexpr functions are usable at compile
> time, such implementation are allowed (but not mandated) by current draft.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_base.h (ranges::__static_size): Define.
> * include/std/ranges (ref_view::size()): For ranges with static
> size return ranges::__static_size<_Range> == 0.
> (ref_view::empty): For ranges with static size, compare it
> against zero.
> * testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc:
> Expect errors from ref_view uses.
> * testsuite/23_containers/inplace_vector/cons/from_range_neg.cc:
> Expect errors from ref_view uses.
> ---
> v2 also expands the empty() method, and adds __static_size helper funciton.
> We could really use some consteval __builtin_unknown_ref(_Tp), that will
> produce reference to unknown object of type _Tp. This will avoid nested
> lambda and instantiating integer_constant.
>
> Tested on x86_64-linux. OK for trunk?
>
> libstdc++-v3/include/bits/ranges_base.h | 9 +++++++++
> libstdc++-v3/include/std/ranges | 18 ++++++++++++++++--
> .../inplace_vector/cons/from_iota_neg.cc | 6 ++----
> .../inplace_vector/cons/from_range_neg.cc | 10 ++++------
> 4 files changed, 31 insertions(+), 12 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/ranges_base.h
> b/libstdc++-v3/include/bits/ranges_base.h
> index fea4100eb09..e61ef75f857 100644
> --- a/libstdc++-v3/include/bits/ranges_base.h
> +++ b/libstdc++-v3/include/bits/ranges_base.h
> @@ -562,6 +562,15 @@ namespace ranges
> template<typename _Tp>
> concept __static_sized_range = sized_range<_Tp> && requires (_Tp& __t)
> { static_cast<char(*)[size_t(ranges::size(__t) >= 0)]>(nullptr); };
> +
> + template<__static_sized_range _Tp>
_Range not _Tp
> + consteval range_size_t<_Tp>
> + __static_size()
> + {
> + auto __conjure = [](_Tp& __t)
__r not __t
> + { return integral_constant<range_size_t<_Tp>, ranges::size(__t)>{}; };
> + return decltype(__conjure(std::declval<_Tp&>()))::value;
> + }
Maybe we could fix the first parameter of integral_constant to size_t and
cast its value to range_size_t afterwards, which should result in fewer
integral_constant instantiations.
It might make sense to add another overload that takes a _Range&
parameter to allow the range type to be deduced. And that overload
could be implemented as a one liner return ranges::size(__r).
We could then define <simd>'s __static_range_size in terms of this.
> #endif // C++26
>
> template<typename _Derived>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 4f2082487d1..c1e8ac3cb17 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -1380,11 +1380,25 @@ namespace views::__adaptor
>
> constexpr bool
> empty() const requires requires { ranges::empty(*_M_r); }
> - { return ranges::empty(*_M_r); }
> + {
> +#if __cplusplus > 202302L
> + if constexpr (__static_sized_range<_Range>)
> + return ranges::__static_size<_Range>() == 0;
> + else
> +#endif
> + return ranges::empty(*_M_r);
> + }
>
> constexpr auto
> size() const requires sized_range<_Range>
> - { return ranges::size(*_M_r); }
> + {
> +#if __cplusplus > 202302L
> + if constexpr (__static_sized_range<_Range>)
> + return ranges::__static_size<_Range>();
> + 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_iota_neg.cc
> b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
> index ae0b96175b7..6b66b5d97ed 100644
> ---
> a/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
> +++
> b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_iota_neg.cc
> @@ -33,14 +33,12 @@ test_all()
> std::inplace_vector<int, 15> tr1(std::from_range, ref_view(m12));
>
> std::inplace_vector<int, 10> tm2(std::from_range, m12); // { dg-error
> "(from here|expansion of)" }
> - // ref_view is not statically sized due pointer dereference
> - std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12));
> + std::inplace_vector<int, 10> tr2(std::from_range, ref_view(m12)); // {
> dg-error "(from here|expansion of)" }
>
> StaticIota<__int128, 0> mm;
>
> std::inplace_vector<int, 10> tm3(std::from_range, mm); // { dg-error
> "(from here|expansion of)" }
> - // ref_view is not statically sized due pointer dereference
> - std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm));
> + std::inplace_vector<int, 10> tr3(std::from_range, ref_view(mm)); // {
> dg-error "(from here|expansion of)" }
> }
>
> // { dg-error "static assertion failed" "" { target *-*-* } 0 }
> 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 48d5b4c56f5..ea187761e5e 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
> @@ -62,16 +62,14 @@ test_all()
>
> test_one(a1); // { dg-error "from here" }
> test_one(s1); // { dg-error "from here" }
> - // ref_view is not statically sized due pointer dereference
> - test_one(ref_view(a1));
> - test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
> + test_one(ref_view(a1)); // { dg-error "from here" }
> + test_one(a5 | std::views::adjacent<5> | std::views::elements<0>); // {
> dg-error "from here" }
> test_one(s5 | 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" }
> - // ref_view is not statically sized due pointer dereference
> - test_five(ref_view(a5));
> - test_five(a7 | std::views::adjacent<3> | std::views::elements<0>);
> + test_five(ref_view(a5)); // { dg-error "from here" }
> + test_five(a7 | std::views::adjacent<3> | std::views::elements<0>); // {
> dg-error "from here" }
> test_five(s7 | std::views::adjacent<3> | std::views::elements<0>); // {
> dg-error "from here" }
> }
>
> --
> 2.53.0
>
>
