Optimize ContinuationWrapper used for support of move-only continuations

After QFuture continuations became non-copyable (see earlier commits),
we have to always use ContinuationWrapper to save the continuations
inside std::function, since it requires the callable to be copyable.
Optimize the wrapper, by storing the callable directly (instead of using
a ref-counted QSharedPointer) and introducing a fake copy-constructor
that makes sure that it's never called.

Pick-to: 6.3 6.2
Change-Id: I0ed5f90ad62ede3b5c6d6e56ef58eb6377122920
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/thread/qfuture_impl.h

@@ -538,11 +538,19 @@ bool Continuation<Function, ResultType, ParentResultType>::execute()
 template<class Function>
 struct ContinuationWrapper
 {
-    ContinuationWrapper(Function &&f) : function(QSharedPointer<Function>::create(std::move(f))) { }
+    ContinuationWrapper(Function &&f) : function(std::move(f)) { }
-    void operator()(const QFutureInterfaceBase &parentData) { (*function)(parentData); }
+    ContinuationWrapper(const ContinuationWrapper &other)
+        : function(std::move(const_cast<ContinuationWrapper &>(other).function))
+    {
+        Q_ASSERT_X(false, "QFuture", "Continuation shouldn't be copied");
+    }
+    ContinuationWrapper(ContinuationWrapper &&other) = default;
+    ContinuationWrapper &operator=(ContinuationWrapper &&) = default;
+
+    void operator()(const QFutureInterfaceBase &parentData) { function(parentData); }
 
 private:
-    QSharedPointer<Function> function;
+    Function function;
 };
 
 template<typename Function, typename ResultType, typename ParentResultType>

src/corelib/thread/qfutureinterface.cpp

@@ -837,7 +837,7 @@ void QFutureInterfaceBasePrivate::setState(QFutureInterfaceBase::State newState)
 
 void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func)
 {
-    setContinuation(func, nullptr);
+    setContinuation(std::move(func), nullptr);
 }
 
 void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func,