QDate: add conversions for std::chrono calendaring classes

std::chrono::year_month_day and related classes offer very
convenient to specify dates.

This patch adds implicit constructors to QDate to support this
convenience, e.g.:

  // YYYY-MM-DD, DD-MM-YYYY, MM-DD-YYYY formats:
  QDate d1 = 1985y / December / 8;
  QDate d2 = 8d / December / 1985;
  QDate d3 = December / 8d / 1985;

  // Indexed weekday:
  QDate d4 = 2000y / January / Monday[0];
  QDate d5 = 2000y / January / Monday[last];

and so on.

These are all implemented using the conversion from the std
calendaring classes to sys_days. Conversions between sys_days
and QDate are also added, since they're basically "for free".

I don't expect "ordinary" users to stumble upon it, but it's
worthy mentioning that std::chrono::year *does* have a year
zero (hence, year_month_day in year 0 or below are offset
by one with the corresponding QDate). I've left a note
in the documentation.

[ChangeLog][QtCore][QDate] QDate (and therefore QDateTime)
is now constructible using the year/month/day/week classes
available in the std::chrono library. Moreover, it now
features conversions from and to std::chrono::sys_days.

Change-Id: I2a4f56423ac7d1469541cbb6a278a65b48878b4a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_time_qdatetime.cpp b/src/corelib/doc/snippets/code/src_corelib_time_qdatetime.cpp
index a477e91548..265f46c067 100644
--- a/src/corelib/doc/snippets/code/src_corelib_time_qdatetime.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_time_qdatetime.cpp
@@ -216,3 +216,23 @@ QString string = "Tuesday, 23 April 12 22:51:41";
 QString format = "dddd, d MMMM yy hh:mm:ss";
 QDateTime valid = QDateTime::fromString(string, format);
 //! [21]
+
+//! [22]
+// 23 April 2012:
+QDate date = std::chrono::year_month_day(std::chrono::year(2012),
+                                         std::chrono::month(4),
+                                         std::chrono::day(23));
+
+// Same, under `using std::chrono` convenience:
+QDate dateWithLiterals1 = 23 / April / 2012y;
+QDate dateWithLiterals2 = 2012y / April / 23;
+
+// Last day of February 2000
+QDate lastDayFeb2020 = 2000y / February / last;
+
+// First Monday of January 2020:
+QDate firstMonday = 2020y / January / Monday[0];
+
+// Last Monday of January 2020:
+QDate lastMonday = 2020y / January / Monday[last];
+//! [22]