QFuture: the result type doesn't have to be a default-constructible

Added asserts to insure the invariant declared in the documentation.
Canceled future object is not valid, hence we don't need to handle
this case separately.

Fixes: QTBUG-83389
Change-Id: Ib0653ef40cd3135574a91740e4ce2c6dc4da8a71
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/corelib/thread/qfutureinterface.h

@@ -339,13 +339,8 @@ inline QList<T> QFutureInterface<T>::results()
 template<typename T>
 T QFutureInterface<T>::takeResult()
 {
-    if (isCanceled()) {
-        exceptionStore().throwPossibleException();
-        return {};
-    }
+    Q_ASSERT(isValid());
 
-    if (!isValid())
-        return {};
     // Note: we wait for all, this is intentional,
     // not to mess with other unready results.
     waitForResult(-1);
@@ -362,13 +357,7 @@ T QFutureInterface<T>::takeResult()
 template<typename T>
 std::vector<T> QFutureInterface<T>::takeResults()
 {
-    if (isCanceled()) {
-        exceptionStore().throwPossibleException();
-        return {};
-    }
-
-    if (!isValid())
-        return {};
+    Q_ASSERT(isValid());
 
     waitForResult(-1);
     std::vector<T> res;

tests/auto/corelib/thread/qfuture/tst_qfuture.cpp

@@ -116,6 +116,8 @@ private slots:
     void runAndTake();
     void resultsReadyAt_data();
     void resultsReadyAt();
+    void takeResultWorksForTypesWithoutDefaultCtor();
+    void canceledFutureIsNotValid();
 private:
     using size_type = std::vector<int>::size_type;
 
@@ -2744,5 +2746,40 @@ void tst_QFuture::resultsReadyAt()
     QCOMPARE(taken, 0b1111);
 }
 
+template <class T>
+auto makeFutureInterface(T &&result)
+{
+    QFutureInterface<T> f;
+
+    f.reportStarted();
+    f.reportResult(std::forward<T>(result));
+    f.reportFinished();
+
+    return f;
+}
+
+void tst_QFuture::takeResultWorksForTypesWithoutDefaultCtor()
+{
+    struct Foo
+    {
+        Foo() = delete;
+        explicit Foo(int i) : _i(i) {}
+
+        int _i = -1;
+    };
+
+    auto f = makeFutureInterface(Foo(42));
+
+    QCOMPARE(f.takeResult()._i, 42);
+}
+void tst_QFuture::canceledFutureIsNotValid()
+{
+    auto f = makeFutureInterface(42);
+
+    f.cancel();
+
+    QVERIFY(!f.isValid());
+}
+
 QTEST_MAIN(tst_QFuture)
 #include "tst_qfuture.moc"