QResultStore::moveResult(): refuse to move from lvalues

The T&& result argument is deduced, therefore result is a Universal
Reference and as such can bind to lvalues, not just rvalues. If passed
an lvalue, the function would happily move away from it, which is not
what the doctor prescribed.

Catch the attempt with a static assertion.

Pick-to: 6.5
Change-Id: Iff8c1abd5dcb1043ed94ba76570be5ba3d6f92da
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/thread/qresultstore.h

@@ -149,6 +149,8 @@ public:
     template <typename T>
     int moveResult(int index, T &&result)
     {
+        static_assert(!std::is_reference_v<T>, "trying to move from an lvalue!");
+
         if (containsValidResultItem(index)) // reject if already present
             return -1;