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

[Tomasz Kaminski]

On Mon, Mar 16, 2026 at 8:33 AM Jonathan Wakely <jwakely.gcc@gmail.com>
wrote:

>
>
> On Thu, 12 Mar 2026, 15:59 Jonathan Wakely, <jwakely@redhat.com> wrote:
>
>> This implements LWG 4242 which was approved in Sofia 2025.
>>
>> Replace the use of the std::decay_t trait with the C++23 auto(x)
>> feature, which avoids instantiating the class template.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/bits/ranges_base.h (distance(It&&, Sent)): Decay
>>         arrays to pointers, as per LWG 4242.
>>         * testsuite/24_iterators/range_operations/lwg4242.cc: New test.
>> ---
>>
>> Lightly tested on x86_64-linux so far, full tests running now.
>>
>
> The tests passed but I forgot to check that Clang allows auto(x) in C++20
> mode. I'll check they before pushing.
>
clang-22 rejects it (https://godbolt.org/z/bY1Gs8xsj), so we will need to
do something like, like
auto __dfirst = __first;
return __last - __difrst;

>
>
>
>
>>  libstdc++-v3/include/bits/ranges_base.h       | 12 ++++++++++-
>>  .../24_iterators/range_operations/lwg4242.cc  | 21 +++++++++++++++++++
>>  2 files changed, 32 insertions(+), 1 deletion(-)
>>  create mode 100644
>> libstdc++-v3/testsuite/24_iterators/range_operations/lwg4242.cc
>>
>> diff --git a/libstdc++-v3/include/bits/ranges_base.h
>> b/libstdc++-v3/include/bits/ranges_base.h
>> index 6cfa6fb6afe6..609e29175f06 100644
>> --- a/libstdc++-v3/include/bits/ranges_base.h
>> +++ b/libstdc++-v3/include/bits/ranges_base.h
>> @@ -989,7 +989,17 @@ namespace ranges
>>        [[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); }
>> +      {
>> +       // GLIBCXX_RESOLVE_LIB_DEFECTS
>> +       // 4242. ranges::distance does not work with volatile iterators
>> +       if constexpr (!is_array_v<remove_reference_t<_It>>)
>> +         return __last - __first;
>> +       else
>> +#pragma GCC diagnostic push
>> +#pragma GCC diagnostic ignored "-Wc++23-extensions" // auto(x)
>> +         return __last - auto(__first);
>> +#pragma GCC diagnostic pop
>> +      }
>>
>>      template<range _Range>
>>        [[nodiscard, __gnu__::__always_inline__]]
>> diff --git
>> a/libstdc++-v3/testsuite/24_iterators/range_operations/lwg4242.cc
>> b/libstdc++-v3/testsuite/24_iterators/range_operations/lwg4242.cc
>> new file mode 100644
>> index 000000000000..ec03be42cbab
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/24_iterators/range_operations/lwg4242.cc
>> @@ -0,0 +1,21 @@
>> +// { dg-do run { target c++20 } }
>> +
>> +// LWG 4242. ranges::distance does not work with volatile iterators
>> +
>> +#include <iterator>
>> +#include <testsuite_hooks.h>
>> +
>> +void
>> +test_lwg4242()
>> +{
>> +  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 );
>> +}
>> +
>> +int main()
>> +{
>> +  test_lwg4242();
>> +}
>> --
>> 2.53.0
>>
>>
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