QFutureInterface: add a warning when an existing continuation is overwritten

... and also extend the documentation to explain this case explicitly.

Fixes: QTBUG-107545
Pick-to: 6.5 6.2
Change-Id: I9414cc677b037989de60e97871485018e5c8a569
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 4 insertions, 0 deletions

diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index ed46052fa7..e656527e04 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -834,6 +834,10 @@ void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInter
     // store the move-only continuation, to guarantee that the associated
     // future's data stays alive.
     if (d->continuationState != QFutureInterfaceBasePrivate::Cleaned) {
+        if (d->continuation) {
+            qWarning() << "Adding a continuation to a future which already has a continuation. "
+                          "The existing continuation is overwritten.";
+        }
         d->continuation = std::move(func);
         d->continuationData = continuationFutureData;
     }