Forbid implicit conversions between QFuture and other types

- Remove the casting operator of QFuture<T> to T. It calls
  QFuture::result(), which may lead to undefined behavior if the user
  has moved the results from QFuture via QFuture::takeResult() before
  trying to do the conversion.

- Disable implicit conversion of QFuture<T> to QFuture<void>, by making
  the constructor explicit. If the users really intend to do the
  conversion, they should do it explicitly.

[ChangeLog][Source-Incompatible Changes][QFuture] Implicit conversions
of QFuture<T> to T and to QFuture<void> have been disabled. Use
QFuture::result() or QFuture::takeResult() where you need to convert
QFuture<T> to T. Use the explicit QFuture<void>(const QFuture<T> &)
constructor to convert QFuture<T> to QFuture<void>.

Change-Id: I153d4137d36365b1611ac934fb3ac2eb667fdd6c
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

2 files changed, 1 insertions, 21 deletions

diff --git a/src/corelib/thread/qfuture.h b/src/corelib/thread/qfuture.h
index c5966f7893..44c5331261 100644
--- a/src/corelib/thread/qfuture.h
+++ b/src/corelib/thread/qfuture.h
@@ -81,7 +81,7 @@ public:
     }
 
     template<typename U, typename V = T, typename = QtPrivate::EnableForVoid<V>>
-    QFuture(const QFuture<U> &other) : d(other.d)
+    explicit QFuture(const QFuture<U> &other) : d(other.d)
     {
     }
 
@@ -96,9 +96,6 @@ public:
     ~QFuture() { }
     QFuture(const QFuture<T> &) { }
     QFuture<T> & operator=(const QFuture<T> &) { }
-
-    // This is required to allow QDoc to find the declaration of operator T().
-    operator T() const;
 #endif
 
     void cancel() { d.cancel(); }
@@ -150,10 +147,6 @@ QT_WARNING_POP
     template<typename U = T, typename = QtPrivate::EnableForNonVoid<U>>
     bool isResultReadyAt(int resultIndex) const { return d.isResultReadyAt(resultIndex); }
 
-    // operator T()
-    template<typename U = T>
-    operator typename std::enable_if_t<!std::is_same_v<U, void>, U>() const { return result(); }
-
     template<typename U = T, typename = QtPrivate::EnableForNonVoid<U>>
     QList<T> results() const { return d.results(); }
 
diff --git a/src/corelib/thread/qfuture.qdoc b/src/corelib/thread/qfuture.qdoc
index 59f80da4af..82b7657016 100644
--- a/src/corelib/thread/qfuture.qdoc
+++ b/src/corelib/thread/qfuture.qdoc
@@ -433,19 +433,6 @@
     \sa resultAt(), resultCount(), takeResult()
 */
 
-/*! \fn template <typename T> QFuture<T>::operator T() const
-
-    Returns the first result in the future. If the result is not immediately
-    available, this function will block and wait for the result to become
-    available. This is a convenience method for calling result() or
-    resultAt(0).
-
-    \note Calling this function leads to undefined behavior if isValid()
-    returns \c false for this QFuture.
-
-    \sa result(), resultAt(), results(), takeResult(), isValid()
-*/
-
 /*! \fn template <typename T> QList<T> QFuture<T>::results() const
 
     Returns all results from the future. If the results are not immediately available,