QDateTime::fromStdTimePoint: fix the constraint

The constraint requires the input time_point's duration to be
convertible to milliseconds. That's inaccurate: we need to check the
duration of the time_point we get from the clock_cast, not the input.
In general, clock_cast is allowed to return arbitrary durations, so
it may not match the input. (It indeed doesn't match for some
standard clocks, like gps_clock.)

Task-number: QTBUG-125587
Change-Id: I5a93317c8cdc0a3cef4acab17c656e2e5dac5d8d
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit e95812df85d98689d51bed92acce51e0e250f854)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qdatetime.cpp

@@ -5352,8 +5352,17 @@ QDateTime QDateTime::currentDateTimeUtc()
     Constructs a datetime representing the same point in time as \a time,
     using Qt::UTC as its specification.
 
-    The clock of \a time must be compatible with \c{std::chrono::system_clock},
+    The clock of \a time must be compatible with
-    and the duration type must be convertible to \c{std::chrono::milliseconds}.
+    \c{std::chrono::system_clock}; in particular, a conversion
+    supported by \c{std::chrono::clock_cast} must exist. After the
+    conversion, the duration type of the result must be convertible to
+    \c{std::chrono::milliseconds}.
+
+    If this is not the case, the caller must perform the necessary
+    clock conversion towards \c{std::chrono::system_clock} and the
+    necessary conversion of the duration type
+    (cast/round/floor/ceil/...) so that the input to this function
+    satisfies the constraints above.
 
     \note This function requires C++20.
 

src/corelib/time/qdatetime.h

@@ -510,6 +510,17 @@ public:
 
 #if __cpp_lib_chrono >= 201907L || defined(Q_QDOC)
 #if __cpp_concepts >= 201907L || defined(Q_QDOC)
+private:
+    // The duration type of the result of a clock_cast<system_clock>.
+    // This duration may differ from the duration of the input.
+    template <typename Clock, typename Duration>
+    using system_clock_cast_duration = decltype(
+        std::chrono::clock_cast<std::chrono::system_clock>(
+            std::declval<const std::chrono::time_point<Clock, Duration> &>()
+        ).time_since_epoch()
+    );
+
+public:
     // Generic clock, as long as it's compatible with us (= system_clock)
     template <typename Clock, typename Duration>
     static QDateTime fromStdTimePoint(const std::chrono::time_point<Clock, Duration> &time)
@@ -517,8 +528,12 @@ public:
             requires(const std::chrono::time_point<Clock, Duration> &t) {
                 // the clock can be converted to system_clock
                 std::chrono::clock_cast<std::chrono::system_clock>(t);
-                // the duration can be converted to milliseconds
+                // after the conversion to system_clock, the duration type
-                requires std::is_convertible_v<Duration, std::chrono::milliseconds>;
+                // we get is convertible to milliseconds
+                requires std::is_convertible_v<
+                    system_clock_cast_duration<Clock, Duration>,
+                    std::chrono::milliseconds
+                >;
             }
     {
         const auto sysTime = std::chrono::clock_cast<std::chrono::system_clock>(time);