[PATCH v3] libstdc++: Make ref_view<R> statically sized if R has static size

Thu May 21 12:20:32 GMT 2026

On Thu, May 21, 2026 at 2:15 PM Patrick Palka <ppalka@redhat.com> wrote:

> On Wed, 20 May 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.
> >
> > Reviewed-by: Patrick Palka <ppalka@redhat.com>
> > Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
> > ---
> > v3 uses __conjures integral_constant<size_t, for all ranges whose size
> > fits insize of size_t, regardless of they size_t.
> >
> >  libstdc++-v3/include/bits/ranges_base.h        | 14 ++++++++++++++
> >  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, 36 insertions(+), 12 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/ranges_base.h
> b/libstdc++-v3/include/bits/ranges_base.h
> > index fea4100eb09..bcfaa56fcab 100644
> > --- a/libstdc++-v3/include/bits/ranges_base.h
> > +++ b/libstdc++-v3/include/bits/ranges_base.h
> > @@ -562,6 +562,20 @@ 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 (for consistency with other helper templates that always take a
> range)
>
> > +    consteval range_size_t<_Tp>
> > +    __static_size()
> > +    {
> > +      auto __conjure = [](_Tp& __t)
>
> __r
>
Changed locally.

>
> > +      {
> > +     if constexpr (ranges::size(__t) <= size_t(-1))
> > +       return integral_constant<size_t, size_t(ranges::size(__t))>{};
> > +     else
> > +       return integral_constant<range_size_t<_Tp>,
> ranges::size(__t)>{};
>
> I didn't realize we can only conditionally use integral_constant<size_t
> here, probably not worth the added noise since we're likely going to get
> the builtin anyway.
>
It will take some time for clang to get it, so I think it is worth doing.
Having
a statically sized range larger than the maximum value of size_t is very
unlikely.
(We have one in our test).

>
> LGTM etiher way
>
> > +      };
> > +      return
> range_size_t<_Tp>(decltype(__conjure(std::declval<_Tp&>()))::value);
> > +    }
> >  #endif // C++26
> >
> >    template<typename _Derived>
> > diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> > index 7d8b37f8c5c..2f11cc2336f 100644
> > --- a/libstdc++-v3/include/std/ranges
> > +++ b/libstdc++-v3/include/std/ranges
> > @@ -1382,11 +1382,25 @@ 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" }
> >  }
> >
> > --
> > 2.54.0
> >
> >
>
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