From 6c504f2519e1180dbcfd77d5bb08b0db9742eeaa Mon Sep 17 00:00:00 2001
From: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Date: Sat, 23 Sep 2023 18:06:15 +0200
Subject: QPointer: also provide a converting assignment operator

d026fad3d962eed0119351cd37f34490e09153fd added converting constructors
for QPointer. This however made converting _assignments_ ambiguous,
introducing a regression for users coming from Qt < 6.6.

This code:

  QPointer<Base> base;
  QPointer<Derived> derived;
  base = derived;

used to convert `derived` to `Derived *` (using the implicit conversion
operator from `QPointer<Derived>` to `Derived *`), and then the
assignment operator for `QPointer<Base>` that took a `Base *`.

The introduction of the conversion constructor in 6.6 makes it possible
to convert `QPointer<Derived>` to `QPointer<Base>`, and then fall back
to the compiler-generated assignment operator for `QPointer<Base>`.

The result is that the code above is now ambiguous and stops compiling.

Fix this by adding a converting assignment operator for QPointer.
I'm only adding the const-lvalue overload because the implementation
requires going through the private QWeakPointer::assign helper. We
cannot copy-assign or move-assign the inner QWeakPointer, as those
assignments require lock()ing the QWeakPointer and that's not possible
on a QObject-tracking QWeakPointer (but cf. QTBUG-117483).

Assigning from a rvalue QPointer would mean calling assign() on
the internal QWeakPointer _and_ clear the incoming QPointer,
and that's strictly worse than the lvalue overload (where we just call
assign()).

Change-Id: I33fb2a22b3d5110284d78e3d7c6cc79a5b73b67b
Pick-to: 6.6 6.6.0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/kernel/qpointer.cpp | 11 +++++++++++
 1 file changed, 11 insertions(+)

(limited to 'src/corelib/kernel/qpointer.cpp')

diff --git a/src/corelib/kernel/qpointer.cpp b/src/corelib/kernel/qpointer.cpp
index c884844db3a..e426c63ae5f 100644
--- a/src/corelib/kernel/qpointer.cpp
+++ b/src/corelib/kernel/qpointer.cpp
@@ -111,6 +111,17 @@
     is convertible to \c{T*}.
 */
 
+/*!
+    \fn template <class T> template <class X> QPointer<T> &QPointer<T>::operator=(const QPointer<X> &other)
+    \since 6.6
+
+    Conversion assignment operator. Makes this guarded pointer guard the
+    same object guarded by \a other.
+
+    \note This operator participates  in overload resolution only if \c{X*}
+    is convertible to \c{T*}.
+*/
+
 /*!
     \fn template <class T> void QPointer<T>::swap(QPointer &other)
     \since 5.6
-- 
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