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>

3 files changed, 37 insertions, 27 deletions

diff --git a/src/corelib/kernel/qtimer.cpp b/src/corelib/kernel/qtimer.cpp
index 6d39233aa7..4a5738a6dc 100644
--- a/src/corelib/kernel/qtimer.cpp
+++ b/src/corelib/kernel/qtimer.cpp
@@ -533,7 +533,7 @@ void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiv
 */
 
 /*!
-    \fn void QTimer::singleShot(std::chrono::duration<Rep, Period> value, const QObject *receiver, const char *member)
+    \fn void QTimer::singleShot(std::chrono::milliseconds msec, const QObject *receiver, const char *member)
     \since 5.8
     \overload
     \reentrant
@@ -545,13 +545,13 @@ void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiv
     create a local QTimer object.
 
     The \a receiver is the receiving object and the \a member is the slot. The
-    time interval is given in the duration object \a value.
+    time interval is given in the duration object \a msec.
 
     \sa start()
 */
 
 /*!
-    \fn void QTimer::singleShot(std::chrono::duration<Rep, Period> value, Qt::TimerType timerType, const QObject *receiver, const char *member)
+    \fn void QTimer::singleShot(std::chrono::milliseconds msec, Qt::TimerType timerType, const QObject *receiver, const char *member)
     \since 5.8
     \overload
     \reentrant
@@ -563,18 +563,18 @@ void QTimer::singleShot(int msec, Qt::TimerType timerType, const QObject *receiv
     create a local QTimer object.
 
     The \a receiver is the receiving object and the \a member is the slot. The
-    time interval is given in the duration object \a value. The \a timerType affects the
+    time interval is given in the duration object \a msec. The \a timerType affects the
     accuracy of the timer.
 
     \sa start()
 */
 
 /*!
-    \fn void QTimer::start(std::chrono::duration<Rep, Period> value)
+    \fn void QTimer::start(std::chrono::milliseconds msec)
     \since 5.8
     \overload
 
-    Starts or restarts the timer with a timeout of duration \a value.
+    Starts or restarts the timer with a timeout of duration \a msec milliseconds.
 
     If the timer is already running, it will be
     \l{QTimer::stop()}{stopped} and restarted.
diff --git a/src/corelib/kernel/qtimer.h b/src/corelib/kernel/qtimer.h
index 1567fe760c..4f934d0367 100644
--- a/src/corelib/kernel/qtimer.h
+++ b/src/corelib/kernel/qtimer.h
@@ -165,38 +165,40 @@ Q_SIGNALS:
 
 public:
 #if QT_HAS_INCLUDE(<chrono>) || defined(Q_QDOC)
-    template <class Rep, class Period>
-    void setInterval(std::chrono::duration<Rep, Period> value)
+    Q_ALWAYS_INLINE
+    void setInterval(std::chrono::milliseconds value)
     {
-        setInterval(std::chrono::duration_cast<std::chrono::milliseconds>(value).count());
+        setInterval(value.count());
     }
 
+    Q_ALWAYS_INLINE
     std::chrono::milliseconds intervalAsDuration() const
     {
         return std::chrono::milliseconds(interval());
     }
 
+    Q_ALWAYS_INLINE
     std::chrono::milliseconds remainingTimeAsDuration() const
     {
         return std::chrono::milliseconds(remainingTime());
     }
 
-    template <class Rep, class Period>
-    static void singleShot(std::chrono::duration<Rep, Period> value, const QObject *receiver, const char *member)
+    Q_ALWAYS_INLINE
+    static void singleShot(std::chrono::milliseconds value, const QObject *receiver, const char *member)
     {
-        singleShot(int(std::chrono::duration_cast<std::chrono::milliseconds>(value).count()), receiver, member);
+        singleShot(int(value.count()), receiver, member);
     }
 
-    template <class Rep, class Period>
-    static void singleShot(std::chrono::duration<Rep, Period> value, Qt::TimerType timerType, const QObject *receiver, const char *member)
+    Q_ALWAYS_INLINE
+    static void singleShot(std::chrono::milliseconds value, Qt::TimerType timerType, const QObject *receiver, const char *member)
     {
-        singleShot(int(std::chrono::duration_cast<std::chrono::milliseconds>(value).count()), timerType, receiver, member);
+        singleShot(int(value.count()), timerType, receiver, member);
     }
 
-    template <class Rep, class Period>
-    void start(std::chrono::duration<Rep, Period> value)
+    Q_ALWAYS_INLINE
+    void start(std::chrono::milliseconds value)
     {
-        start(int(std::chrono::duration_cast<std::chrono::milliseconds>(value).count()));
+        start(int(value.count()));
     }
 #endif
 
@@ -215,15 +217,13 @@ private:
                                const QObject *receiver, QtPrivate::QSlotObjectBase *slotObj);
 
 #if QT_HAS_INCLUDE(<chrono>)
-    template <class Rep, class Period>
-    static Qt::TimerType defaultTypeFor(std::chrono::duration<Rep, Period> interval)
-    { return defaultTypeFor(int(std::chrono::duration_cast<std::chrono::milliseconds>(interval).count())); }
+    static Qt::TimerType defaultTypeFor(std::chrono::milliseconds interval)
+    { return defaultTypeFor(int(interval.count())); }
 
-    template <class Rep, class Period>
-    static void singleShotImpl(std::chrono::duration<Rep, Period> interval, Qt::TimerType timerType,
+    static void singleShotImpl(std::chrono::milliseconds interval, Qt::TimerType timerType,
                                const QObject *receiver, QtPrivate::QSlotObjectBase *slotObj)
     {
-        singleShotImpl(int(std::chrono::duration_cast<std::chrono::milliseconds>(interval).count()),
+        singleShotImpl(int(interval.count()),
                        timerType, receiver, slotObj);
     }
 #endif
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);