QDeadlineTimer: make the (Qt::TimerType) ctor explicit

Qt::TimerType is not a faithful representation of a QDeadlineTimer.
It's missing the actual deadline, and so should not be allowed to
participate in implicit conversions.

Consider whether you'd like to see such code:

    foo.setTimeout(Qt::PreciseTimer); // HU???

So split the old (TimerType = CoarseTimer) ctor into an explicit
default ctor (applying NSDMI to make it =default'able) and a new
explicit (TimerType) ctor.

[ChangeLog][Potentially Source-Incompatible Changes][QtCore] The
QDeadlineTimer(Qt::TimerType) constructor is no longer implicit. To
keep old source working, wrap the Qt::TimerType argument in
QDeadlineTimer{}. This is backwards-compatible with older Qt versions.

[ChangeLog][QtCore][QDeadlineTimer] The (Qt::TimerType) constructor is
now explicit (was: implicit).

Found in API-review.

Change-Id: I72aaaad5201bd288a6fd6af398573aaa9d0eda5a
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 17f2b6d2e9379d7edaf5acd17a6547ca46b12f2b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qdeadlinetimer.cpp

@@ -131,10 +131,12 @@ static qint64 add_saturate(qint64 t1, Duration1 dur, Durations... extra)
 */
 
 /*!
+    \fn QDeadlineTimer::QDeadlineTimer()
     \fn QDeadlineTimer::QDeadlineTimer(Qt::TimerType timerType)
 
     Constructs an expired QDeadlineTimer object. For this object,
-    remainingTime() will return 0.
+    remainingTime() will return 0. If \a timerType is not set, then the object
+    will use the \l{Qt::CoarseTimer}{coarse} \l{QDeadlineTimer#Timer types}{timer type}.
 
     The timer type \a timerType may be ignored, since the timer is already
     expired. Similarly, for optimization purposes, this function will not

src/corelib/kernel/qdeadlinetimer.h

@@ -26,7 +26,8 @@ public:
     enum class ForeverConstant { Forever };
     static constexpr ForeverConstant Forever = ForeverConstant::Forever;
 
-    constexpr QDeadlineTimer(Qt::TimerType type_ = Qt::CoarseTimer) noexcept
+    constexpr QDeadlineTimer() noexcept = default;
+    constexpr explicit QDeadlineTimer(Qt::TimerType type_) noexcept
         : type(type_) {}
     constexpr QDeadlineTimer(ForeverConstant, Qt::TimerType type_ = Qt::CoarseTimer) noexcept
         : t1((std::numeric_limits<qint64>::max)()), type(type_) {}
@@ -142,7 +143,7 @@ private:
 #if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
     unsigned t2 = 0;
 #endif
-    unsigned type;
+    unsigned type = Qt::CoarseTimer;
 
     qint64 rawRemainingTimeNSecs() const noexcept;
 };