[LWG3294] zoned_time deduction guides misinterprets string/char*

[tomaszkam]

Original comment:

Do TimeZonePtrOrName deduction guides.

E-mail thread:
[isocpp-lib] zoned_time issues

[tomaszkam]

Tim's comment:

It seems that the TimeZonePtr deduction guides could use some
constraining, since right now zoned_time("America/New_York",
local_seconds{...}) is going to try to use a const char* as a time
zone pointer.

[tomaszkam]

Discussion:
Current deduction guide for zoned_time for the following expression:

zoned_time zpc("America/New_York", std::chrono::system_clock::now());
zoned_time zps(std::string("America/New_York"), std::chrono::system_clock::now());

will attempt to produce an zoned_time instance with either char const* or std::string as time zone implementation. This is caused by the fact that uncostrained TimeZonePtr deduction guide template will produce better candidates and will be selected by overload resolution.

Proposed resolution merges deduction std::string_view/TimeZonePtr deduction guides into one guide, that deduces time_zone const* for any type convertible to string_view. This is necessary to override the deduction from TimeZonePtr candidates.

In addition, we disable deduction from string_view constructors, that would produce better candidates than deduction guides and create zoned_time instances with durations coarser than seconds (causing similiar issue as LWG3232):

std::chrono::local_time lh(10h);
std::chrono::zoned_time zt1("Europe/Helsinki", lh);
std::chrono::zoned_time zt2(std::string("Europe/Helsinki"), lh);
std::chrono::zoned_time zt3(std::string_view("Europe/Helsinki"), lh);

Without disabling the deduction from string_view constructor, the zt3 type would be deduced to zoned_time&;lt;hours>, with the proposed change types zt1, zt2 and zt3 are consistently deduced as zoned_time&;lt;secons>.

Finally, the wording eliminates uncessary zoned_time<Duration> guide (covered by zoned_time<Duration, TimeZonePtr2>).

The change was implemented in the example implementation. The dedicated test can be found here: https://github.com/HowardHinnant/date/blob/master/test/tz_test/zoned_time_deduction.pass.cpp.

Proposed wording:
Apply the following changes to [time.zone.zonedtime.overview]:

zoned_time() -> zoned_time;

template<class Duration>
  zoned_time(sys_time<Duration>)
    -> zoned_time<common_type_t<Duration, seconds>>;


template<typename TimeZonePtrOrName>
using time_zone_representation =
  conditional_t<is_convertible_v<TimeZonePtrOrName, string_view>, time_zone const*, remove_cv_ref<TimeZonePtrOrName>>;  //exposition only

template<class TimeZonePtrOrName>
zoned_time(TimeZonePtrOrName&&)
  -> zoned_time<seconds, time_zone_representation<TimeZonePtr>>;

template<class TimeZonePtrOrName, class Duration>
zoned_time(TimeZonePtrOrName&&, sys_time<Duration>)
  -> zoned_time<common_type_t<Duration, seconds>, time_zone_representation<TimeZonePtrOrName>>;

template<class TimeZonePtrOrName, class Duration>
zoned_time(TimeZonePtrOrName&&, local_time<Duration>, choose = choose::earliest)
  -> zoned_time<common_type_t<Duration, seconds>, time_zone_representation<TimeZonePtrOrName>>;


template<class TimeZonePtr, class Duration>
zoned_time(TimeZonePtr, zoned_time<Duration>, choose = choose::earliest)
  -> zoned_time<common_type_t<Duration, seconds>, TimeZonePtr>;

zoned_time(string_view) -> zoned_time<seconds>;

template<class Duration>
zoned_time(string_view, sys_time<Duration>)
  -> zoned_time<common_type_t<Duration, seconds>>;

template<class Duration>
zoned_time(string_view, local_time<Duration>, choose = choose::earliest)
  -> zoned_time<common_type_t<Duration, seconds>>;


template<class Duration, class TimeZonePtrOrName, class TimeZonePtr2>
zoned_time(TimeZonePtrOrName&&, zoned_time<Duration, TimeZonePtr2>, choose = choose::earliest)
  -> zoned_time<Duration, time_zone_representation<TimeZonePtrOrName>>;                               

Add the following paragraphs at the end of section [time.zone.zonedtime.overview]:
Constructor of zoned_time that accepts a string_view as first parameter does not particpate in class template argument deduction ([over.match.class.deduct]).

[tomaszkam]

@HowardHinnant:
Created an pull request with test: HowardHinnant#475
My test have found few issues in the current implementation (see commits description).