On Fri, 31 Jul 2026 at 10:21, Jonathan Wakely <[email protected]> wrote:
>
> 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.
Yes, but there's no reason to preserve the bug in the release branches
when it's a trivial fix.
> > >>
> > >>
> > >> 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.
And that's what the code did previously, because read_unsigned sets
failbit, and then we reached the 'break'.
But I think my patch is still OK because __read_unsigned(2) will set
failbit if it sees "@@" instead of a number. That means even if we
don't break immediately, we won't extract any more characters and will
break when the next read fails. I don't think we need to
micro-optimize for the case of garbage input, so if we break
immediately or break a few nanoseconds later, it's fine.
I'll update the test to include "2026-07-31 xx:xx", "%F %R" cases.
>
> > >>
> > >> 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
> > >>
