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WIP [libc++][ranges] P3052R2: view_interface::at() #205012

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2 changes: 2 additions & 0 deletions libcxx/docs/FeatureTestMacroTable.rst
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Expand Up @@ -308,6 +308,8 @@ Status
---------------------------------------------------------- -----------------
``__cpp_lib_variant`` ``202106L``
---------------------------------------------------------- -----------------
``__cpp_lib_view_interface`` ``202606L``
---------------------------------------------------------- -----------------
**C++23**
----------------------------------------------------------------------------
``__cpp_lib_adaptor_iterator_pair_constructor`` ``202106L``
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2 changes: 1 addition & 1 deletion libcxx/docs/Status/Cxx29Papers.csv
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Expand Up @@ -4,7 +4,7 @@
"`P2414R12 <https://wg21.link/P2414R12>`__","Pointer lifetime-end zap proposed solutions","2026-06 (Brno)","","","`#204392 <https://github.com/llvm/llvm-project/issues/204392>`__","Voted as a Defect Report."
"`P3319R6 <https://wg21.link/P3319R6>`__","Add an ``iota`` object for ``simd`` (and more)","2026-06 (Brno)","","","`#204393 <https://github.com/llvm/llvm-project/issues/204393>`__",""
"`P3798R1 <https://wg21.link/P3798R1>`__","The unexpected in ``std::expected``","2026-06 (Brno)","","","`#204394 <https://github.com/llvm/llvm-project/issues/204394>`__",""
"`P3052R2 <https://wg21.link/P3052R2>`__","``view_interface::at()``","2026-06 (Brno)","","","`#204395 <https://github.com/llvm/llvm-project/issues/204395>`__",""
"`P3052R2 <https://wg21.link/P3052R2>`__","``view_interface::at()``","2026-06 (Brno)","|Complete|,"23","`#204395 <https://github.com/llvm/llvm-project/issues/204395>`__",""
"`P4206R0 <https://wg21.link/P4206R0>`__","Revert string support in ``std::constant_wrapper``","2026-06 (Brno)","","","`#203336 <https://github.com/llvm/llvm-project/issues/203336>`__","To be applied as a Defect Report."
"`P3395R6 <https://wg21.link/P3395R6>`__","Fix encoding issues and add a formatter for ``std::error_code``","2026-06 (Brno)","","","`#204396 <https://github.com/llvm/llvm-project/issues/204396>`__",""
"`P3505R4 <https://wg21.link/P3505R4>`__","Fix the default floating-point representation in ``std::format``","2026-06 (Brno)","","","`#204397 <https://github.com/llvm/llvm-project/issues/204397>`__","To be applied as a Defect Report."
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22 changes: 22 additions & 0 deletions libcxx/include/__ranges/view_interface.h
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Expand Up @@ -15,6 +15,7 @@
#include <__concepts/same_as.h>
#include <__config>
#include <__iterator/concepts.h>
#include <__iterator/distance.h>
#include <__iterator/iterator_traits.h>
#include <__iterator/prev.h>
#include <__memory/pointer_traits.h>
Expand All @@ -25,6 +26,7 @@
#include <__type_traits/is_class.h>
#include <__type_traits/make_unsigned.h>
#include <__type_traits/remove_cv.h>
#include <stdexcept>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
Expand Down Expand Up @@ -159,6 +161,26 @@ class view_interface {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) operator[](range_difference_t<_RARange> __index) const {
return ranges::begin(__derived())[__index];
}

template <random_access_range _RARange = _Derived>
requires sized_range<_RARange> // freestanding-deleted
[[nodiscard]] constexpr decltype(auto) at(range_difference_t<_RARange> __index) {
if (__index < 0 || __index >= ranges::distance(__derived())) {
std::__throw_out_of_range(
"Precondition `0 <= __index < distance()` not satisfied. `.at(__index)` called with out-of-bounds index.");
}
return (*this)[__index];
}

template <random_access_range _RARange = const _Derived>
requires sized_range<_RARange> // freestanding-deleted
[[nodiscard]] constexpr decltype(auto) at(range_difference_t<_RARange> __index) const {
if (__index < 0 || __index >= ranges::distance(__derived())) {
std::__throw_out_of_range(
"Precondition `0 <= __index < distance()` not satisfied. `.at(__index)` called with out-of-bounds index.");
}
return (*this)[__index];
}
};

} // namespace ranges
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2 changes: 2 additions & 0 deletions libcxx/include/version
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Expand Up @@ -289,6 +289,7 @@ __cpp_lib_unwrap_ref 201811L <functional>
__cpp_lib_variant 202306L <variant>
202106L // C++20
202102L // C++17
__cpp_lib_view_interface 202606L <ranges>
__cpp_lib_void_t 201411L <type_traits>

*/
Expand Down Expand Up @@ -487,6 +488,7 @@ __cpp_lib_void_t 201411L <type_traits>
# define __cpp_lib_unwrap_ref 201811L
# undef __cpp_lib_variant
# define __cpp_lib_variant 202106L
# define __cpp_lib_view_interface 202606L
#endif

#if _LIBCPP_STD_VER >= 23
Expand Down
5 changes: 5 additions & 0 deletions libcxx/test/libcxx/ranges/range.utility/view.interface/nodiscard.verify.cpp
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Expand Up @@ -67,4 +67,9 @@ void test() {
v[Diff{0}];
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
std::as_const(v)[Diff{0}];

// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
v.at(Diff{0});
// expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
std::as_const(v).at(Diff{0});
}
1 change: 1 addition & 0 deletions libcxx/test/libcxx/transitive_includes/cxx23.csv
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Expand Up @@ -11,6 +11,7 @@ algorithm iosfwd
algorithm limits
algorithm optional
algorithm ratio
algorithm stdexcept
algorithm tuple
algorithm version
any cstdint
Expand Down
1 change: 1 addition & 0 deletions libcxx/test/libcxx/transitive_includes/cxx26.csv
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Expand Up @@ -11,6 +11,7 @@ algorithm iosfwd
algorithm limits
algorithm optional
algorithm ratio
algorithm stdexcept
algorithm tuple
algorithm version
any cstdint
Expand Down
33 changes: 33 additions & 0 deletions ...td/language.support/support.limits/support.limits.general/ranges.version.compile.pass.cpp
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Expand Up @@ -80,6 +80,10 @@
# error "__cpp_lib_ranges_zip should not be defined before c++23"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

#elif TEST_STD_VER == 14

# ifdef __cpp_lib_default_template_type_for_algorithm_values
Expand Down Expand Up @@ -142,6 +146,10 @@
# error "__cpp_lib_ranges_zip should not be defined before c++23"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

#elif TEST_STD_VER == 17

# ifdef __cpp_lib_default_template_type_for_algorithm_values
Expand Down Expand Up @@ -204,6 +212,10 @@
# error "__cpp_lib_ranges_zip should not be defined before c++23"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

#elif TEST_STD_VER == 20

# ifdef __cpp_lib_default_template_type_for_algorithm_values
Expand Down Expand Up @@ -269,6 +281,13 @@
# error "__cpp_lib_ranges_zip should not be defined before c++23"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++20"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++20"
# endif

#elif TEST_STD_VER == 23

# ifdef __cpp_lib_default_template_type_for_algorithm_values
Expand Down Expand Up @@ -385,6 +404,13 @@
# error "__cpp_lib_ranges_zip should have the value 202110L in c++23"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++23"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++23"
# endif

#elif TEST_STD_VER > 23

# if !defined(_LIBCPP_VERSION)
Expand Down Expand Up @@ -516,6 +542,13 @@
# error "__cpp_lib_ranges_zip should have the value 202110L in c++26"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++26"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++26"
# endif

#endif // TEST_STD_VER > 23

// clang-format on
33 changes: 33 additions & 0 deletions ...d/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp
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Expand Up @@ -944,6 +944,10 @@
# error "__cpp_lib_variant should not be defined before c++17"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

# ifdef __cpp_lib_void_t
# error "__cpp_lib_void_t should not be defined before c++17"
# endif
Expand Down Expand Up @@ -1946,6 +1950,10 @@
# error "__cpp_lib_variant should not be defined before c++17"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

# ifdef __cpp_lib_void_t
# error "__cpp_lib_void_t should not be defined before c++17"
# endif
Expand Down Expand Up @@ -3137,6 +3145,10 @@
# error "__cpp_lib_variant should have the value 202102L in c++17"
# endif

# ifdef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should not be defined before c++20"
# endif

# ifndef __cpp_lib_void_t
# error "__cpp_lib_void_t should be defined in c++17"
# endif
Expand Down Expand Up @@ -4595,6 +4607,13 @@
# error "__cpp_lib_variant should have the value 202106L in c++20"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++20"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++20"
# endif

# ifndef __cpp_lib_void_t
# error "__cpp_lib_void_t should be defined in c++20"
# endif
Expand Down Expand Up @@ -6281,6 +6300,13 @@
# error "__cpp_lib_variant should have the value 202106L in c++23"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++23"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++23"
# endif

# ifndef __cpp_lib_void_t
# error "__cpp_lib_void_t should be defined in c++23"
# endif
Expand Down Expand Up @@ -8303,6 +8329,13 @@
# error "__cpp_lib_variant should have the value 202306L in c++26"
# endif

# ifndef __cpp_lib_view_interface
# error "__cpp_lib_view_interface should be defined in c++26"
# endif
# if __cpp_lib_view_interface != 202606L
# error "__cpp_lib_view_interface should have the value 202606L in c++26"
# endif

# ifndef __cpp_lib_void_t
# error "__cpp_lib_void_t should be defined in c++26"
# endif
Expand Down
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