https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111163
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
With this patch:
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -507,6 +507,16 @@ namespace __format
if constexpr (__is_specialization_of<_Tp, chrono::hh_mm_ss>)
__is_neg = __t.is_negative();
+ if constexpr (chrono::__is_duration_v<_Tp>)
+ if constexpr (_Tp::period::num != 1)
+ {
+ // Check conversion to seconds will not overflow.
+ chrono::seconds::rep __s{};
+ if (__builtin_mul_overflow(__t.count(), _Tp::period::num,
&__s))
+ __throw_format_error("chrono format error: integer overflow "
+ "converting duration to seconds");
+ }
+
auto __print_sign = [&__is_neg, &__out] {
if constexpr (chrono::__is_duration_v<_Tp>
|| __is_specialization_of<_Tp, chrono::hh_mm_ss>)
The testcase throws:
terminate called after throwing an instance of 'std::format_error'
what(): chrono format error: integer overflow converting duration to seconds
Aborted (core dumped)
This isn't ideal as there are some uses that wouldn't overflow, e.g.
std::format("{:%j}", std::chrono::days::max())
This doesn't require any conversion and so could be formatted. But I'm not sure
I care about this case, it's pretty unlikely to ever be needed.
If we did the overflow checks at the point of conversion then we'd be doing
them multiple times for a format string like "%H:%M:%S". But then I suppose
we're already converting the duration to hh_mm_ss three times there anyway. The
patch in PR 110739 comment 2 would help with that, as the _S_hms call might be
subject to common subexpression elimination, and the checks would only be done
once. We could also just call _S_hms once up-front when we have a type that can
make use of it, and then reuse the result as needed.
And the patch above doesn't handle all cases anyway, because this will still
overflow:
std::format("{:%S}", std::chrono::sys_days{std::chrono::days::max()});
So maybe we just need a checked duration_cast<days>, floor<days>, floor<weeks>,
and the check for conversion to seconds for use with hh_mm_ss.
