[PATCH 1/2] libstdc++: static_assert that static sized range size is less than inplace_vector capacity
Jonathan Wakely
jwakely.gcc@gmail.com
Wed Mar 18 22:17:14 GMT 2026
On Wed, 18 Mar 2026 at 15:15, Tomasz Kamiński <tkaminsk@redhat.com> wrote:
>
> Resolves LWG 4396. Improve inplace_vector(from_range_t, R&& rg)
>
> This patch introduces a new __static_sized_range internal concept, for checking
> if is static (independed on the object and know at compile time). The
> implementation requires validating that ranges::size(__r) can be used as
> constant template argument. For that check we use non_type_t, that is now
"nontype_t" without the first underscore
> always make available in C++26.
"make" -> "made"
OK with those changes
>
> The patch does not put extra considerations towards support integer-class size
> types, as none of standard statically-sized ranges uses such type as
> distance_type. If the need arrives we may later want to make __max_size_type
> and __max_diff_type structural.
>
> The test case illustrates that views applied to span<T, N> are still statically
> sized, but once applied to array<T, N> are not. This is caused by the fact that
> ref_view stores a pointer to array, and deference of unknown pointers does not
> produce reference to unknown, and simply yield non-constant expressions
> (unknown pointer is not equivalent to pointer to unknown, as it may be null).
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/ranges_base.h (__static_sized_range): Define.
> * include/bits/utility.h (std::non_type_t, std::non_type)
> [__cplusplus > 202302L]: Change include guard.
> * include/std/inplace_vector (inplace_vector(std::from_range, __Rg&&)):
> Add static_asserts checking range size.
> * testsuite/23_containers/inplace_vector/cons/from_range_neg.cc: New test.
> ---
> CC Mathias as he has similar concept in the SIMD patches, and will need
> to rebase (and provide second one for __static_sized_range<_Rg, N>).
>
> Testing on x86_64-linux. The *inplace_vector* test already passed.
> OK for trunk when all test passes?
>
> libstdc++-v3/include/bits/ranges_base.h | 8 +++
> libstdc++-v3/include/bits/utility.h | 2 +-
> libstdc++-v3/include/std/inplace_vector | 16 ++++-
> .../inplace_vector/cons/from_range_neg.cc | 70 +++++++++++++++++++
> 4 files changed, 93 insertions(+), 3 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
>
> diff --git a/libstdc++-v3/include/bits/ranges_base.h b/libstdc++-v3/include/bits/ranges_base.h
> index 519927f5613..3f4a0c09dbb 100644
> --- a/libstdc++-v3/include/bits/ranges_base.h
> +++ b/libstdc++-v3/include/bits/ranges_base.h
> @@ -558,6 +558,14 @@ namespace ranges
> template<sized_range _Range>
> using range_size_t = decltype(ranges::size(std::declval<_Range&>()));
>
> +#if __cplusplus > 202302L
> + template<typename _Tp>
> + concept __static_sized_range = sized_range<_Tp>
> + && requires(_Tp&& __r) {
> + typename nontype_t<ranges::size(__r)>;
> + };
> +#endif
> +
> template<typename _Derived>
> requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
> class view_interface; // defined in <bits/ranges_util.h>
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
> index bd6b18d54dd..c667ce488d6 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -312,7 +312,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> inline constexpr sorted_equivalent_t sorted_equivalent{};
> #endif
>
> -#if __glibcxx_function_ref // >= C++26
> +#if __cplusplus > 202302L
> template<auto>
> struct nontype_t
> {
> diff --git a/libstdc++-v3/include/std/inplace_vector b/libstdc++-v3/include/std/inplace_vector
> index c22a9e1f9b4..bbe3594cd0e 100644
> --- a/libstdc++-v3/include/std/inplace_vector
> +++ b/libstdc++-v3/include/std/inplace_vector
> @@ -40,7 +40,7 @@
> #include <initializer_list>
> #include <bits/stdexcept_throw.h>
> #include <bits/range_access.h>
> -#include <bits/ranges_base.h> // borrowed_iterator_t, __detail::__container_compatible_range
> +#include <bits/ranges_base.h> // borrowed_iterator_t, __detail::__container_compatible_range, __static_sized_range
> #include <bits/ranges_util.h> // subrange
> #include <bits/ranges_uninitialized.h>
> #include <bits/stl_construct.h>
> @@ -119,7 +119,14 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> constexpr
> inplace_vector(from_range_t, _Rg&& __rg)
> : inplace_vector()
> - { append_range(__rg); }
> + {
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4396. Improve inplace_vector(from_range_t, R&& rg)
> + if constexpr (ranges::__static_sized_range<_Rg>)
> + static_assert(ranges::size(__rg) <= _Nm);
> +
> + append_range(__rg);
> + }
>
> constexpr
> inplace_vector(initializer_list<_Tp> __il)
> @@ -961,6 +968,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> constexpr
> inplace_vector(from_range_t, _Rg&& __rg)
> {
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4396. Improve inplace_vector(from_range_t, R&& rg)
> + if constexpr (ranges::__static_sized_range<_Rg>)
> + static_assert(ranges::size(__rg) == 0);
> +
> if (ranges::begin(__rg) != ranges::end(__rg))
> __throw_bad_alloc();
> }
> 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
> new file mode 100644
> index 00000000000..cf87129da51
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/inplace_vector/cons/from_range_neg.cc
> @@ -0,0 +1,70 @@
> +// { dg-do compile { target c++26 } }
> +
> +#include <array>
> +#include <ranges>
> +#include <span>
> +#include <inplace_vector>
> +
> +template<typename Range0>
> +void
> +test_zero(Range0&& r0)
> +{
> + std::inplace_vector<int, 0> f0(std::from_range, r0);
> + std::inplace_vector<int, 1> f1(std::from_range, r0);
> + std::inplace_vector<int, 5> f5(std::from_range, r0);
> + std::inplace_vector<int, 7> f7(std::from_range, r0);
> +}
> +
> +template<typename Range1>
> +void
> +test_one(Range1&& r1)
> +{
> + std::inplace_vector<int, 0> f0(std::from_range, r1); // { dg-error "required from" }
> + std::inplace_vector<int, 1> f1(std::from_range, r1);
> + std::inplace_vector<int, 5> f5(std::from_range, r1);
> + std::inplace_vector<int, 7> f7(std::from_range, r1);
> +}
> +
> +template<typename Range5>
> +void
> +test_five(Range5&& r5)
> +{
> + std::inplace_vector<int, 0> f0(std::from_range, r5); // { dg-error "required from" }
> + std::inplace_vector<int, 1> f1(std::from_range, r5); // { dg-error "required from" }
> + std::inplace_vector<int, 3> f3(std::from_range, r5); // { dg-error "required from" }
> + std::inplace_vector<int, 5> f5(std::from_range, r5);
> + std::inplace_vector<int, 7> f7(std::from_range, r5);
> +}
> +
> +void
> +test_all()
> +{
> + std::array<int, 0> a0;
> + std::span<int, 0> s0(a0);
> + std::array<int, 1> a1;
> + std::span<int, 1> s1(a1);
> + std::array<int, 5> a5;
> + std::span<int, 5> s5(a5);
> + std::array<int, 7> a7;
> + std::span<int, 7> s7(a7);
> +
> + test_zero(a0);
> + test_zero(s0);
> + test_zero(std::views::empty<int>);
> + test_one(s5 | std::views::adjacent<7> | std::views::elements<0>);
> + test_one(a5 | std::views::adjacent<7> | std::views::elements<0>);
> +
> + 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_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>);
> +}
> +
> +// { dg-error "static assertion failed" "" { target *-*-* } 0 }
> --
> 2.53.0
>
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