QFuture is tricky in the sense that it can store multiple results. The users might write the code like: QList<int> values{1, 2, 3}; auto f = QtConcurrent::mapped(values, func) .then([](auto val) { otherFunc(val); }); with the expectation that the lambda will be called on each mapped value of the original input container. However, that is not true. QtConcurrent::mapped() returns a QFuture which has multiple results, and in this case only the first result will be passed to the then() continuation. We cannot detect this problem at compile-time, because we do not know how many results will the QFuture hold. So, add at least a runtime warning. The warning is added in a new logging category, so that the users who really need this behavior can disable it. The warning is implemented as an exported out-of-line method for two reasons: * To avoid code duplication for each template parameter of CompactContinuation. * To avoid exposing a Qt-specific logging category into a public header. Such design, however, prevents us from cherry-picking the change to older branches. Fixes: QTBUG-133522 Change-Id: I3344b5228b50e9c9d68d0c57961bbc969f566bb9 Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>