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: 6.6.0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 6c504f2519e1180dbcfd77d5bb08b0db9742eeaa)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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

src/corelib/kernel/qpointer.h

@@ -43,6 +43,13 @@ public:
     QPointer(const QPointer<X> &other) noexcept
         : wp(other.wp.internalData(), true) {}
 
+    template <typename X, if_convertible<X> = true>
+    QPointer &operator=(const QPointer<X> &other)
+    {
+        wp.assign(other.data());
+        return *this;
+    }
+
 #ifdef Q_QDOC
     // Stop qdoc from complaining about missing function
     ~QPointer();

tests/auto/corelib/kernel/qpointer/tst_qpointer.cpp

@@ -57,6 +57,19 @@ void tst_QPointer::conversion()
         QCOMPARE_EQ(pio.get(), &file);
         QCOMPARE_EQ(pio, pf);
         QCOMPARE_EQ(pio.get(), pf.get());
+
+        // reset
+        pio = nullptr;
+        QCOMPARE_EQ(pio, nullptr);
+        QCOMPARE_EQ(pio.get(), nullptr);
+
+        // copy-assignment
+        QCOMPARE_EQ(pf, &file);
+        pio = pf;
+        QCOMPARE_EQ(pio, &file);
+        QCOMPARE_EQ(pio.get(), &file);
+        QCOMPARE_EQ(pio, pf);
+        QCOMPARE_EQ(pio.get(), pf.get());
     }
     // move-conversion:
     {
@@ -67,6 +80,16 @@ void tst_QPointer::conversion()
         QCOMPARE_EQ(pf, nullptr);
         QCOMPARE_EQ(pio, &file);
         QCOMPARE_EQ(pio.get(), &file);
+
+        // reset
+        pio = nullptr;
+        QCOMPARE_EQ(pio, nullptr);
+        QCOMPARE_EQ(pio.get(), nullptr);
+
+        // move-assignment
+        pio = QPointer<QFile>(&file);
+        QCOMPARE_EQ(pio, &file);
+        QCOMPARE_EQ(pio.get(), &file);
     }
 }