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

On Fri, 31 Jul 2026 at 10:13, Jonathan Wakely <[email protected]> wrote:
>
> 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.

Actually, thinking about this further, I think if __val == -1 is true,
we do want to break. That means we failed to parse any number at all.
It makes sense to ignore 99:99 but for garbage like @@:?? the parse
should fail.

> >>
> >>                           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
> >>