QFuture: fix QtFuture::connect corner-cases

Connecting to nullptr, or connecting to a non-signal PMF, would result
in a QFuture which would never finish. Catch these cases and handle
them.

Windows+MSVC for some reason fails the test. I can't entirely understand
why, so I've marked it as XFAIL, with QTBUG-101761 to track it.

Change-Id: I314980e7e9b7156d8cddd3b33d5cbf1d0bcd6116
Pick-to: 6.2 6.3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/thread/qfuture_impl.h

@@ -880,6 +880,11 @@ static QFuture<ArgsType<Signal>> connect(Sender *sender, Signal signal)
     using ArgsType = ArgsType<Signal>;
     QFutureInterface<ArgsType> promise;
     promise.reportStarted();
+    if (!sender) {
+        promise.reportCanceled();
+        promise.reportFinished();
+        return promise.future();
+    }
 
     using Connections = std::pair<QMetaObject::Connection, QMetaObject::Connection>;
     auto connections = std::make_shared<Connections>();
@@ -910,6 +915,12 @@ static QFuture<ArgsType<Signal>> connect(Sender *sender, Signal signal)
                                               });
     }
 
+    if (!connections->first) {
+        promise.reportCanceled();
+        promise.reportFinished();
+        return promise.future();
+    }
+
     connections->second =
             QObject::connect(sender, &QObject::destroyed, sender, [promise, connections]() mutable {
                 QObject::disconnect(connections->first);

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

@@ -3768,6 +3768,36 @@ void tst_QFuture::signalConnect()
         QVERIFY(!future.isCanceled());
         QVERIFY(future.isValid());
     }
+
+    // Connect to nullptr
+    {
+        SenderObject *sender = nullptr;
+        auto future = QtFuture::connect(sender, &SenderObject::intArgSignal);
+        QVERIFY(future.isFinished());
+        QVERIFY(future.isCanceled());
+        QVERIFY(!future.isValid());
+    }
+
+    // Connect to non-signal
+    {
+        SenderObject sender;
+
+#if defined(Q_CC_MSVC) && !defined(Q_CC_CLANG)
+#define EXPECT_FUTURE_CONNECT_FAIL() QEXPECT_FAIL("", "QTBUG-101761, test fails on Windows/MSVC", Continue)
+#else
+        QTest::ignoreMessage(QtWarningMsg, "QObject::connect: signal not found in SenderObject");
+#define EXPECT_FUTURE_CONNECT_FAIL()
+#endif
+
+        auto future = QtFuture::connect(&sender, &SenderObject::emitNoArg);
+        EXPECT_FUTURE_CONNECT_FAIL();
+        QVERIFY(future.isFinished());
+        EXPECT_FUTURE_CONNECT_FAIL();
+        QVERIFY(future.isCanceled());
+        EXPECT_FUTURE_CONNECT_FAIL();
+        QVERIFY(!future.isValid());
+#undef EXPECT_FUTURE_CONNECT_FAIL
+    }
 }
 
 void tst_QFuture::waitForFinished()