On Tue, 19 May 2026, Patrick Palka wrote:
> 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;
return __conjure(std::declval<_Tp&>()).value;
is slightly shorter (assuming it works with our current p2280 impl)
> > + }
>
> 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
> >
> >
