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Implement LWG-4257: Stream insertion for chrono::local_time should be constrained #5895

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Implement LWG-4257: Stream insertion for chrono::local_time should be constrained #5895

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16589dc
Constrain operator<< for chrono::local_time.
vmichal Nov 21, 2025
dbb77b9
Code review feedback.
StephanTLavavej Nov 24, 2025
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4 changes: 3 additions & 1 deletion stl/inc/chrono
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Original file line number Diff line number Diff line change
Expand Up @@ -5294,7 +5294,9 @@ namespace chrono {
}

_EXPORT_STD template <class _CharT, class _Traits, class _Duration>
basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const local_time<_Duration>& _Val) {
basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const local_time<_Duration>& _Val)
requires requires { _Os << sys_time<_Duration>{_Val.time_since_epoch()}; }
{
return _Os << sys_time<_Duration>{_Val.time_since_epoch()};
}

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37 changes: 37 additions & 0 deletions tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp
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Expand Up @@ -20,6 +20,8 @@

#include <timezone_data.hpp>

// Extended to test LWG-4257 "Stream insertion for chrono::local_time should be constrained"

using namespace std;
using namespace chrono;

Expand Down Expand Up @@ -1061,7 +1063,42 @@ void test_locale() {
assert(stream(year_month_weekday_last{2021y / May / Tuesday[last]}) == STR("2021/Mai/Di[last]"));
}

template <typename T>
concept ostream_insertable = requires(ostream& o, const T& t) { o << t; };

template <typename Dur>
void check_stream_insertion_operator_for_duration() {
if constexpr (ostream_insertable<sys_time<Dur>>) {
ostringstream oss;
oss << sys_time<Dur>{};
assert(oss.str() == "1970-01-01 00:00:00");
}

if constexpr (ostream_insertable<local_time<Dur>>) {
ostringstream oss;
oss << local_time<Dur>{};
assert(oss.str() == "1970-01-01 00:00:00");
}
}

// Test based on example in LWG-4257
void check_stream_insertion_operator() {
// operator<< is constrained such that it does not participate when underlying duration has floating-point rep
using ok_dur = duration<long long>;
using bad_dur = duration<double>;

static_assert(ostream_insertable<sys_time<ok_dur>>);
static_assert(ostream_insertable<local_time<ok_dur>>);
check_stream_insertion_operator_for_duration<ok_dur>();

static_assert(!ostream_insertable<sys_time<bad_dur>>);
static_assert(!ostream_insertable<local_time<bad_dur>>);
check_stream_insertion_operator_for_duration<bad_dur>();
}

void test() {
check_stream_insertion_operator();

test_parse_conversion_spec<char>();
test_parse_conversion_spec<wchar_t>();

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