[aizu-m]

UBSan, clang -fsanitize=undefined, when formatting a time point with a floating-point rep:

__chrono/duration.h:75:63: runtime error: 1e+300 is outside the range of representable values of type 'long'
__chrono/duration.h:75:63: runtime error: nan is outside the range of representable values of type 'long long'

Repro:

using fsec = std::chrono::duration<double>;
fmt::format("{:%T}", fmt::sys_time<fsec>(fsec(1e300)));
fmt::format("{:%T}", fmt::sys_time<fsec>(fsec(NAN)));  // prints "00:00:00an"

The mixed integer <-> float overload of detail::duration_cast delegates to std::chrono::duration_cast, so to_time_t and the sys_time / local_time formatters convert the seconds count with an unchecked static_cast. Only the float to integral direction can overflow, so the overload is split: integer to float keeps the plain cast, and float to integral range-checks the count before casting (which also rejects NaN) and throws like the other checked overloads, with the same boundary handling as to_nonnegative_int (#4802). is_similar_arithmetic_type had no remaining users and is removed.

[vitaut]

Floating-point-to-integer conversion truncates toward zero, so values slightly below the minimum are also valid. For example, converting -0.5 to unsigned produces 0, but this check rejects it. Could we check std::trunc(count) instead and add a regression test?

[aizu-m]

Good catch. The check was on the raw count, so -0.5 to unsigned threw even though the cast gives 0. Same at the signed end: INT_MIN - 0.5 was rejected but truncates to INT_MIN.

Switched it to check std::trunc(count) in c9b6642. NaN still fails the comparison, so that path is unchanged. Added chrono_test.float_to_integral_duration_cast for the -0.5 to unsigned, INT_MIN - 0.5 and INT_MAX + 0.5 cases, plus -1 and INT_MIN - 1 still throwing. It fails on the previous revision and passes now, with the rest of chrono-test clean under UBSan.

[vitaut]

One remaining concern: the change removes support for custom duration representations. The previous overload handled any combination of representation types that wasn't integral-to-integral or floating-point-to-floating-point. The new overloads only handle built-in arithmetic types.

I think we should preserve the original fallback for other representation types.

[aizu-m]

Right, the split dropped every rep that isn't a built-in arithmetic type. A duration<custom_rep> to seconds cast stopped compiling with "no matching function for call to 'duration_cast'".

Restored the original fallback in f501601. It now covers everything that isn't similar-arithmetic and isn't float to integral, so custom reps and integer to float go through std::chrono::duration_cast as before. Only float to integral takes the range-checked path. is_similar_arithmetic_type is back.

Added chrono_test.custom_rep_duration_cast. It fails to compile on the previous revision and passes now. chrono-test is clean under UBSan.