Re: [PATCH] libstdc++: Make chrono::parse accept out-of-range values that aren't needed [PR126364]

On Fri, 31 Jul 2026 at 06:48, Tomasz Kaminski <[email protected]> wrote:
>
>
>
> On Thu, Jul 30, 2026 at 1:41 PM Jonathan Wakely <[email protected]> wrote:
>>
>> When parsing a time with %R or %T we should ignore out of range hours
>> and minutes if the type being parsed doesn't need them, e.g. when
>> parsing a chrono::year_month_day from "2026-07-29 99:99:99" we do not
>> set failbit, and should continue parsing after the invalid hours and
>> minutes.
>>
>> Because we were short circuiting as soon as we saw "99" (in either
>> field) we didn't parse to the end of the %R or %T field, and then could
>> set failbit if there were any subsequent characters or flags to parse.
>>
>> The fix is to only short-circuit when setting failbit, and continue
>> parsing otherwise.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         PR libstdc++/126364
>>         * include/bits/chrono_io.h (_Parser::operator()) <R>: Only break
>>         early when setting failbit.
>>         * testsuite/std/time/parse/126364.cc: New test.
>> ---
>>
>> Tested x86_64-linux.
>
> LGTM with only one small change.
>>
>>
>> This should be backported to 14, 15 and 16 as well.
>
> Agreed; this is OK to backport as the patch is trivial.
> However, this is C++20 feature, so we do not strictly need to if I understand
> correctly.
>>
>>
>>  libstdc++-v3/include/bits/chrono_io.h         | 12 ++++---
>>  .../testsuite/std/time/parse/126364.cc        | 34 +++++++++++++++++++
>>  2 files changed, 42 insertions(+), 4 deletions(-)
>>  create mode 100644 libstdc++-v3/testsuite/std/time/parse/126364.cc
>>
>> diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
>> index c5170368f82f..51e07ff55593 100644
>> --- a/libstdc++-v3/include/bits/chrono_io.h
>> +++ b/libstdc++-v3/include/bits/chrono_io.h
>> @@ -4737,8 +4737,10 @@ namespace __detail
>>                       if (__val == -1 || __val > 23) [[unlikely]]
>>                         {
>
> You do not need these braces anymore, and can put it directly under the above if
> (We want to merge them into one, as the condition mentioned above is not unlikely)

I assume you mean we do *not* want to merge them into one.

I'll make that change, thanks.

>>
>>                           if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
>> -                           __err |= ios_base::failbit;
>> -                         break;
>> +                           {
>> +                             __err |= ios_base::failbit;
>> +                             break;
>> +                           }
>>                         }
>>                       if (!__read_chr(':')) [[unlikely]]
>>                         break;
>> @@ -4748,8 +4750,10 @@ namespace __detail
>>                       if (__val == -1 || __val > 60) [[unlikely]]
>>                         {
>
> Same here.
>>
>>                           if ((_M_need & _ChronoParts::_TimeOfDay) != 0)
>> -                           __err |= ios_base::failbit;
>> -                         break;
>> +                           {
>> +                             __err |= ios_base::failbit;
>> +                             break;
>> +                           }
>>                         }
>>                       __min = minutes(__val);
>>
>> diff --git a/libstdc++-v3/testsuite/std/time/parse/126364.cc b/libstdc++-v3/testsuite/std/time/parse/126364.cc
>> new file mode 100644
>> index 000000000000..74a94a6b84b8
>> --- /dev/null
>> +++ b/libstdc++-v3/testsuite/std/time/parse/126364.cc
>> @@ -0,0 +1,34 @@
>> +// { dg-do run { target c++20 } }
>> +
>> +// Bug 126364 - chrono::from_stream %T and %R short circuit on out of range
>> +// values even when it doesn't fail the parse
>> +
>> +#include <chrono>
>> +#include <sstream>
>> +#include <testsuite_hooks.h>
>> +
>> +using namespace std::chrono;
>> +
>> +void
>> +test_pr126364()
>> +{
>> +  auto check = [](const char* input, char const* fmt) {
>> +      std::istringstream is(input);
>> +      year_month_day ymd{};
>> +      return from_stream(is, fmt, ymd).good() && ymd.ok();
>> +  };
>> +
>> +  VERIFY( check("2019-09-05T25:36:57Z", "%FT%TZ") );  // hour 25 only
>> +  VERIFY( check("2019-09-05T20:99:57Z", "%FT%TZ") );  // minute 99 only
>> +  VERIFY( check("2019-09-05T20:36:99Z", "%FT%TZ") );  // second 99 only
>> +  VERIFY( check("2019-09-05T25:36:57Z", "%FT%T") );   // hour 25 only, no Z
>> +  VERIFY( check("2019-09-05T20:99:57Z", "%FT%T") );   // minute 99 only, no Z
>> +  VERIFY( check("2019-09-05T20:36:99Z", "%FT%T") );   // second 99 only, no Z
>> +  VERIFY( check("2019-09-05 25", "%F %H") );          // %H out of range alone
>> +  VERIFY( check("2019-09-05 99", "%F %M") );          // %M out of range alone
>> +}
>> +
>> +int main()
>> +{
>> +  test_pr126364();
>> +}
>> --
>> 2.55.0
>>