[PATCH 1/2] libstdc++: static_assert that static sized range size is less than inplace_vector capacity
Tomasz Kamiński
tkaminsk@redhat.com
Wed Mar 18 15:06:57 GMT 2026
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
always make available in C++26.
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
More information about the Libstdc++
mailing list