QtConcurrent::run: support non default-constructible return types

The QtConcurrent::RunFunctionTask class keeps a variable to store the
result of QtConcurrent::run when it becomes available, so that it can be
reported afterwards. This requires the result type to be
default-constructible. However there's no need in storing the result, it
can be reported immediately after it becomes available.

Pick-to: 6.1 6.2
Fixes: QTBUG-95214
Change-Id: I95f3dbff0ab41eaa81b104a8834b37d10a0d193a
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/concurrent/qtconcurrentrunbase.h

@@ -117,37 +117,15 @@ public:
             promise.reportException(QUnhandledException(std::current_exception()));
         }
 #endif
-
-        reportResult();
-
         promise.reportFinished();
     }
 
 protected:
     virtual void runFunctor() = 0;
-    virtual void reportResult() {}
 
     QFutureInterface<T> promise;
 };
 
-template <typename T>
-class RunFunctionTask : public RunFunctionTaskBase<T>
-{
-protected:
-    void reportResult() override
-    {
-        if constexpr (std::is_move_constructible_v<T>)
-            this->promise.reportAndMoveResult(std::move(result));
-        else if constexpr (std::is_copy_constructible_v<T>)
-            this->promise.reportResult(result);
-    }
-
-    T result;
-};
-
-template <>
-class RunFunctionTask<void> : public RunFunctionTaskBase<void> {};
-
 } //namespace QtConcurrent
 
 #endif //Q_QDOC

src/concurrent/qtconcurrentstoredfunctioncall.h

@@ -138,7 +138,7 @@ template <class ...Types>
 using DecayedTuple = std::tuple<std::decay_t<Types>...>;
 
 template <class Function, class ...Args>
-struct StoredFunctionCall : public RunFunctionTask<InvokeResultType<Function, Args...>>
+struct StoredFunctionCall : public RunFunctionTaskBase<InvokeResultType<Function, Args...>>
 {
     StoredFunctionCall(DecayedTuple<Function, Args...> &&_data)
         : data(std::move(_data))
@@ -152,10 +152,17 @@ protected:
             return std::invoke(function, args...);
         };
 
-        if constexpr (std::is_void_v<InvokeResultType<Function, Args...>>)
+        if constexpr (std::is_void_v<InvokeResultType<Function, Args...>>) {
             std::apply(invoke, std::move(data));
-        else
-            this->result = std::apply(invoke, std::move(data));
+        } else {
+            auto result = std::apply(invoke, std::move(data));
+
+            using T = InvokeResultType<Function, Args...>;
+            if constexpr (std::is_move_constructible_v<T>)
+                this->promise.reportAndMoveResult(std::move(result));
+            else if constexpr (std::is_copy_constructible_v<T>)
+                this->promise.reportResult(result);
+        }
     }
 
 private:

tests/auto/concurrent/qtconcurrentrun/tst_qtconcurrentrun.cpp

@@ -64,6 +64,7 @@ private slots:
     void moveOnlyType();
     void crefFunction();
     void customPromise();
+    void nonDefaultConstructibleValue();
 };
 
 void light()
@@ -1564,6 +1565,17 @@ void tst_QtConcurrentRun::customPromise()
     QCOMPARE(p.future().progressMaximum(), 10);
 }
 
+void tst_QtConcurrentRun::nonDefaultConstructibleValue()
+{
+    struct NonDefaultConstructible
+    {
+        explicit NonDefaultConstructible(int v) : value(v) { }
+        int value = 0;
+    };
+
+    auto future = QtConcurrent::run([] { return NonDefaultConstructible(42); });
+    QCOMPARE(future.result().value, 42);
+}
 
 QTEST_MAIN(tst_QtConcurrentRun)
 #include "tst_qtconcurrentrun.moc"

tests/auto/corelib/thread/qfuturewatcher/tst_qfuturewatcher.cpp

@@ -229,12 +229,12 @@ void tst_QFutureWatcher::canceled()
     future.waitForFinished();
 }
 
-class IntTask : public RunFunctionTask<int>
+class IntTask : public RunFunctionTaskBase<int>
 {
 public:
     void runFunctor() override
     {
-        result = 10;
+        promise.reportResult(10);
     }
 };
 
@@ -463,7 +463,7 @@ void tst_QFutureWatcher::disconnectRunningFuture()
 }
 
 const int maxProgress = 100000;
-class ProgressEmitterTask : public RunFunctionTask<void>
+class ProgressEmitterTask : public RunFunctionTaskBase<void>
 {
 public:
     void runFunctor() override
@@ -493,7 +493,7 @@ void tst_QFutureWatcher::tooMuchProgress()
 }
 
 template <typename T>
-class ProgressTextTask : public RunFunctionTask<T>
+class ProgressTextTask : public RunFunctionTaskBase<T>
 {
 public:
     void runFunctor() override