QThread: fix UB (invalid enum value) on Private::Priority

The Unix code stores an additional flag, ThreadPriorityResetFlag, in
the Policy enum, but ubsan does not approve:

  qthread_unix.cpp:303:30: runtime error: load of value 2147483648, which is not a valid value for type 'Priority'
  qthread_unix.cpp:304:75: runtime error: load of value 2147483648, which is not a valid value for type 'Priority'

Fix by making the variable of std::underlying_type_t<Priority>.

The masking and unmasking code can now be simplified, too.

In the Windows version, replace some switch targets with equivalent
ones to keep -Wswitch-like warnings, though I hasten to note that both
switches use a default case, so have anyway implicitly disabled said
warning.

Pick-to: 6.3 6.2 5.15
Change-Id: Ie4ea7d05e2928d2755ad12d36535197f85493191
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/corelib/thread/qthread_win.cpp b/src/corelib/thread/qthread_win.cpp
index 820ffa1149..4b144ebda4 100644
--- a/src/corelib/thread/qthread_win.cpp
+++ b/src/corelib/thread/qthread_win.cpp
@@ -457,7 +457,7 @@ void QThread::start(Priority priority)
 
     int prio;
     d->priority = priority;
-    switch (d->priority) {
+    switch (priority) {
     case IdlePriority:
         prio = THREAD_PRIORITY_IDLE;
         break;
@@ -584,7 +584,7 @@ void QThreadPrivate::setPriority(QThread::Priority threadPriority)
 
     int prio;
     priority = threadPriority;
-    switch (priority) {
+    switch (threadPriority) {
     case QThread::IdlePriority:
         prio = THREAD_PRIORITY_IDLE;
         break;