QTaggedPointer: disable operator= with an empty initializer list

Given a QTaggedPointer, users may write

  taggedPtr = {};

to mean "reset it". This is error-prone: due to overload resolution,
this actually ends up calling QTaggedPointer<T>::operator=(T *),
which changes the pointer but *not* the tag, and not the implicitly
declared QTaggedPointer<T>:operator=(const QTaggedPointer<T> &)
which would reset both pointer and tag.

Given the idiomatic usage of {} is indeed to perform a full reset (cf.
std::exchange(obj, {}), std::take, etc.), work around this by disabling
the operator= overload for pointers in case an initializer list is
passed. In other words, make `={}` fall back to the implicitly
declared overload.

Note, this breaks some usages, such as

  taggedPtr = {rawPtr};

but at least we get a compile error for these, and they don't look
common at all.

[ChangeLog][QtCore][QTaggedPointer] The operator assignment
taking a raw pointer has been reimplemented in order to avoid
subtle issues when assigning `{}` to a QTaggedPointer. This will
cause code that assigns a braced-init-list to a QTaggedPointer object
to stop compiling (for instance, `tagPtr = {ptr}` is now ill-formed).

Change-Id: I5e572a9b0f119ddb2df17f1797934933dff2ba7b
Task-number: QTBUG-106070
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 21 insertions, 2 deletions

diff --git a/src/corelib/tools/qtaggedpointer.h b/src/corelib/tools/qtaggedpointer.h
index 504645993a..a08912c4a5 100644
--- a/src/corelib/tools/qtaggedpointer.h
+++ b/src/corelib/tools/qtaggedpointer.h
@@ -72,11 +72,30 @@ public:
         return !isNull();
     }
 
-    QTaggedPointer &operator=(T *other) noexcept
+#ifdef Q_QDOC
+    QTaggedPointer &operator=(T *other) noexcept;
+#else
+    // Disables the usage of `ptr = {}`, which would go through this operator
+    // (rather than using the implicitly-generated assignment operator).
+    // The operators have different semantics: the ones here leave the tag intact,
+    // the implicitly-generated one overwrites it.
+    template <typename U,
+              std::enable_if_t<std::is_convertible_v<U *, T *>, bool> = false>
+    QTaggedPointer &operator=(U *other) noexcept
+    {
+        T *otherT = other;
+        d = reinterpret_cast<quintptr>(otherT) | (d & tagMask());
+        return *this;
+    }
+
+    template <typename U,
+              std::enable_if_t<std::is_null_pointer_v<U>, bool> = false>
+    QTaggedPointer &operator=(U) noexcept
     {
-        d = reinterpret_cast<quintptr>(other) | (d & tagMask());
+        d = reinterpret_cast<quintptr>(static_cast<T *>(nullptr)) | (d & tagMask());
         return *this;
     }
+#endif
 
     static constexpr Tag maximumTag() noexcept
     {