Add support for unwrapping QFuture<QFuture<T>>

Added QFuture::unwrap() for unwrapping the future nested inside
QFuture<QFuture<T>>. QTBUG-86725 suggests doing the unwrapping
automatically inside .then(), but this will change the return type
of .then() that used to return QFuture<QFuture<T>> and might cause
SC breaks. Apart from that, QFuture::unwrap() might be helpful in
general, for asynchronous computations that return a nested QFuture.

[ChangeLog][QtCore][QFuture] Added QFuture::unwrap() for unwrapping the
future nested inside QFuture<QFuture<T>>.

Task-number: QTBUG-86725
Change-Id: I8886743aca261dca46f62d9dfcaead4a141d3dc4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 20 insertions, 0 deletions

diff --git a/src/corelib/thread/qfuture.h b/src/corelib/thread/qfuture.h
index 7aed38be16..c450221769 100644
--- a/src/corelib/thread/qfuture.h
+++ b/src/corelib/thread/qfuture.h
@@ -191,6 +191,14 @@ QT_WARNING_POP
     template<class Function, typename = std::enable_if_t<std::is_invocable_r_v<T, Function>>>
     QFuture<T> onCanceled(QObject *context, Function &&handler);
 
+#if !defined(Q_CLANG_QDOC)
+    template<class U = T, typename = std::enable_if_t<QtPrivate::isQFutureV<U>>>
+    auto unwrap();
+#else
+    template<class U>
+    QFuture<U> unwrap();
+#endif
+
     class const_iterator
     {
     public:
@@ -317,6 +325,8 @@ private:
     template<typename ResultType>
     friend struct QtPrivate::WhenAnyContext;
 
+    friend struct QtPrivate::UnwrapHandler;
+
     using QFuturePrivate =
             std::conditional_t<std::is_same_v<T, void>, QFutureInterfaceBase, QFutureInterface<T>>;
 
@@ -431,6 +441,16 @@ QFuture<T> QFuture<T>::onCanceled(QObject *context, Function &&handler)
     return promise.future();
 }
 
+template<class T>
+template<class U, typename>
+auto QFuture<T>::unwrap()
+{
+    if constexpr (QtPrivate::isQFutureV<typename QtPrivate::Future<T>::type>)
+        return QtPrivate::UnwrapHandler::unwrapImpl(this).unwrap();
+    else
+        return QtPrivate::UnwrapHandler::unwrapImpl(this);
+}
+
 inline QFuture<void> QFutureInterface<void>::future()
 {
     return QFuture<void>(this);