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libstdc++: Make ranges::distance work with volatile iterators (LWG 4242)

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This implements LWG 4242 which was approved in Sofia 2025.

I don't think the change from static_cast<const decay_t<I>&> to just
static_cast<decay_t<I>> is observable, but it doesn't hurt. What fixes
the problem identified in the issue is the is_array_v check, which
avoids the static_cast entirely for volatile-qualified iterators.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (distance(It&&, Sent)): Only decay
	arrays to pointers when the type is actually an array, as per
	LWG 4242.
	* testsuite/24_iterators/range_operations/distance.cc: Add test
	for LWG 4242.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
This commit is contained in:
Jonathan Wakely
2026-03-12 15:15:57 +00:00
committed by Jonathan Wakely
parent a65cafbc52
commit c351a240ad
2 changed files with 28 additions and 3 deletions
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13
libstdc++-v3/include/bits/ranges_base.h
View File

@@ -969,8 +969,6 @@ namespace ranges
struct __distance_fn final
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3664. LWG 3392 broke std::ranges::distance(a, a+3)
template<typename _It, sentinel_for<_It> _Sent>
requires (!sized_sentinel_for<_Sent, _It>)
constexpr iter_difference_t<_It>
@@ -985,11 +983,20 @@ namespace ranges
return __n;
}
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 3392. cannot be used on a move-only iterator with a sized sentinel
// 3664. LWG 3392 broke std::ranges::distance(a, a+3)
// 4242. ranges::distance does not work with volatile iterators
template<typename _It, sized_sentinel_for<decay_t<_It>> _Sent>
[[nodiscard, __gnu__::__always_inline__]]
constexpr iter_difference_t<decay_t<_It>>
operator()(_It&& __first, _Sent __last) const
{ return __last - static_cast<const decay_t<_It>&>(__first); }
{
if constexpr (!is_array_v<remove_reference_t<_It>>)
return __last - __first;
else
return __last - static_cast<decay_t<_It>>(__first);
}
template<range _Range>
[[nodiscard, __gnu__::__always_inline__]]

18
libstdc++-v3/testsuite/24_iterators/range_operations/distance.cc
View File

@@ -154,6 +154,23 @@ test06()
VERIFY( std::ranges::distance(a+3, a) == -3 );
}
void
test_lwg4242()
{
// LWG 4242. ranges::distance does not work with volatile iterators
int arr[] = {1, 2, 3};
int* volatile ptr = arr;
auto d1 = std::distance(ptr, arr + 3);
auto d2 = std::ranges::distance(ptr, arr + 3);
VERIFY( d1 == d2 );
// This is not part of LWG 4242 but it doesn't hurt to check it anyway:
volatile int vol_arr[1]{};
auto d3 = std::distance(vol_arr, vol_arr + 1);
auto d4 = std::ranges::distance(vol_arr, vol_arr + 1);
VERIFY( d3 == d4 );
}
int
main()
{
@@ -163,4 +180,5 @@ main()
test04();
test05();
test06();
test_lwg4242();
}