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[libc++] Add [[nodiscard]] to std::prev and std::next #109550

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Merged
merged 8 commits into from
Sep 28, 2024
Merged

[libc++] Add [[nodiscard]] to std::prev and std::next #109550

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ea7e0a7
[libc++] Add [[nodiscard]] to std::prev and std::next
marcauberer Sep 21, 2024
e988c95
Format
marcauberer Sep 21, 2024
9f8f94e
Add tests
marcauberer Sep 25, 2024
7d6bd72
Fix unused variable
marcauberer Sep 25, 2024
e2dfff2
Fix problem in test
marcauberer Sep 25, 2024
0d7fd0b
Fix CI
marcauberer Sep 27, 2024
9f467cc
Ignore return value for existing test cases
marcauberer Sep 28, 2024
a1ac81a
Format
marcauberer Sep 28, 2024
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11 changes: 6 additions & 5 deletions libcxx/include/__iterator/next.h
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Expand Up @@ -25,7 +25,7 @@
_LIBCPP_BEGIN_NAMESPACE_STD

template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
next(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
// Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
// Note that this check duplicates the similar check in `std::advance`.
Expand All @@ -43,25 +43,26 @@ next(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
namespace ranges {
struct __next {
template <input_or_output_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
++__x;
return __x;
}

template <input_or_output_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
ranges::advance(__x, __n);
return __x;
}

template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, _Sp __bound_sentinel) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, _Sp __bound_sentinel) const {
ranges::advance(__x, __bound_sentinel);
return __x;
}

template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip
operator()(_Ip __x, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const {
ranges::advance(__x, __n, __bound_sentinel);
return __x;
}
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9 changes: 5 additions & 4 deletions libcxx/include/__iterator/prev.h
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Expand Up @@ -25,7 +25,7 @@
_LIBCPP_BEGIN_NAMESPACE_STD

template <class _InputIter, __enable_if_t<__has_input_iterator_category<_InputIter>::value, int> = 0>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
[[__nodiscard__]] inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 _InputIter
prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n = 1) {
// Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation.
// Note that this check duplicates the similar check in `std::advance`.
Expand All @@ -42,19 +42,20 @@ prev(_InputIter __x, typename iterator_traits<_InputIter>::difference_type __n =
namespace ranges {
struct __prev {
template <bidirectional_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x) const {
--__x;
return __x;
}

template <bidirectional_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n) const {
ranges::advance(__x, -__n);
return __x;
}

template <bidirectional_iterator _Ip>
_LIBCPP_HIDE_FROM_ABI constexpr _Ip operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Ip
operator()(_Ip __x, iter_difference_t<_Ip> __n, _Ip __bound_iter) const {
ranges::advance(__x, -__n, __bound_iter);
return __x;
}
Expand Down
18 changes: 15 additions & 3 deletions libcxx/test/libcxx/diagnostics/iterator.nodiscard.verify.cpp
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Expand Up @@ -15,12 +15,24 @@
#include <iterator>
#include <vector>

#include "test_macros.h"

void test() {
std::vector<int> container;
int c_array[] = {1, 2, 3};
std::initializer_list<int> initializer_list;

std::empty(container); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::empty(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::empty(initializer_list); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::empty(container); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::empty(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::empty(initializer_list); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::prev(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::next(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
#if TEST_STD_VER >= 20
std::ranges::prev(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::ranges::prev(container.end(), 2); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::ranges::next(container.end(), 2, container.begin()); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::ranges::next(c_array); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::ranges::next(container.begin(), 2); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
std::ranges::next(container.end(), 1, container.end()); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
#endif
}
4 changes: 2 additions & 2 deletions libcxx/test/libcxx/iterators/assert.next.pass.cpp
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Expand Up @@ -23,8 +23,8 @@
int main(int, char**) {
int a[] = {1, 2, 3};
forward_iterator<int *> it(a+1);
std::next(it, 1); // should work fine
std::next(it, 0); // should work fine
(void)std::next(it, 1); // should work fine
(void)std::next(it, 0); // should work fine
TEST_LIBCPP_ASSERT_FAILURE(std::next(it, -1), "Attempt to next(it, n) with negative n on a non-bidirectional iterator");

return 0;
Expand Down
10 changes: 5 additions & 5 deletions libcxx/test/libcxx/iterators/assert.prev.pass.cpp
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Expand Up @@ -24,13 +24,13 @@ int main(int, char**) {
int a[] = {1, 2, 3};

bidirectional_iterator<int *> bidi(a+1);
std::prev(bidi, -1); // should work fine
std::prev(bidi, 0); // should work fine
std::prev(bidi, 1); // should work fine
(void)std::prev(bidi, -1); // should work fine
(void)std::prev(bidi, 0); // should work fine
(void)std::prev(bidi, 1); // should work fine

forward_iterator<int *> it(a+1);
std::prev(it, -1); // should work fine
std::prev(it, 0); // should work fine
(void)std::prev(it, -1); // should work fine
(void)std::prev(it, 0); // should work fine
TEST_LIBCPP_ASSERT_FAILURE(std::prev(it, 1), "Attempt to prev(it, n) with a positive n on a non-bidirectional iterator");

return 0;
Expand Down
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