QTimer: don't circumvent <chrono> safety net

By templating on the <chrono> types and unconditionally using
duration_cast to coerce the duration into a milliseconds, we
violate a principal design rule of <chrono>, namely that non-
narrowing conversions are implicit, but narrowing conversions
need duration_cast. By accepting any duration, we allow non-
sensical code such as

   QTimer::singleShot(10us, ...)

to compile, which is misleading, since it's actually a zero-
timeout timer.

Overloading a non-template with a template also has adverse
effects: it breaks qOverload().

Fix by replacing the function templates with functions that
just take std::chrono::milliseconds. This way, benign code
such as

    QTimer::singleShot(10s, ...)
    QTimer::singleShot(10min, ...)
    QTimer::singleShot(1h, ...)

work as expected, but attempts to use sub-millisecond
resolution fails to compile / needs an explicit user-
provided duration_cast.

To allow future extension to more precise timers, forcibly
inline the functions, so they don't partake in the ABI of the
class and we can later support sub-millisecond resolution by
simply taking micro- or nano- instead of milliseconds.

Change-Id: I12c9a98bdabefcd8ec18a9eb09f87ad908d889de
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp b/tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp
index 28df01cc16..fe97695d19 100644
--- a/tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp
+++ b/tests/auto/corelib/kernel/qtimer/tst_qtimer.cpp
@@ -216,6 +216,16 @@ void tst_QTimer::remainingTimeDuringActivation()
     }
 }
 
+namespace {
+
+#if QT_HAS_INCLUDE(<chrono>)
+    template <typename T>
+    std::chrono::milliseconds to_ms(T t)
+    { return std::chrono::duration_cast<std::chrono::milliseconds>(t); }
+#endif
+
+} // unnamed namespace
+
 void tst_QTimer::basic_chrono()
 {
 #if !QT_HAS_INCLUDE(<chrono>)
@@ -225,7 +235,7 @@ void tst_QTimer::basic_chrono()
     using namespace std::chrono;
     TimerHelper helper;
     QTimer timer;
-    timer.setInterval(nanoseconds(0));
+    timer.setInterval(to_ms(nanoseconds(0)));
     timer.start();
     QCOMPARE(timer.intervalAsDuration().count(), milliseconds::rep(0));
     QCOMPARE(timer.remainingTimeAsDuration().count(), milliseconds::rep(0));
@@ -248,7 +258,7 @@ void tst_QTimer::basic_chrono()
     QVERIFY(helper.count > oldCount);
 
     helper.count = 0;
-    timer.start(microseconds(200000));
+    timer.start(to_ms(microseconds(200000)));
     QCOMPARE(timer.intervalAsDuration().count(), milliseconds::rep(200));
     QTest::qWait(50);
     QCOMPARE(helper.count, 0);
@@ -864,7 +874,7 @@ void tst_QTimer::singleShot_chrono()
     QCOMPARE(nhelper.count, 1);
 
     int count = 0;
-    QTimer::singleShot(microseconds(0), CountedStruct(&count));
+    QTimer::singleShot(to_ms(microseconds(0)), CountedStruct(&count));
     QCoreApplication::processEvents();
     QCOMPARE(count, 1);