1 files changed, 29 insertions, 8 deletions

diff --git a/src/corelib/tools/qtaggedpointer.h b/src/corelib/tools/qtaggedpointer.h
index 504645993a..6c467d59f8 100644
--- a/src/corelib/tools/qtaggedpointer.h
+++ b/src/corelib/tools/qtaggedpointer.h
@@ -21,7 +21,7 @@ namespace QtPrivate {
         static_assert((alignment & (alignment - 1)) == 0,
             "Alignment of template parameter must be power of two");
 
-        static constexpr quint8 tagBits = QtPrivate::qConstexprCountTrailingZeroBits(alignment);
+        static constexpr quint8 tagBits = quint8{QtPrivate::qConstexprCountTrailingZeroBits(alignment)};
         static_assert(tagBits > 0,
             "Alignment of template parameter does not allow any tags");
 
@@ -43,10 +43,10 @@ public:
     static constexpr quintptr tagMask() { return QtPrivate::TagInfo<T>::alignment - 1; }
     static constexpr quintptr pointerMask() { return ~tagMask(); }
 
-    constexpr QTaggedPointer() noexcept : d(0) {}
-    constexpr QTaggedPointer(std::nullptr_t) noexcept : QTaggedPointer() {}
+    Q_NODISCARD_CTOR constexpr QTaggedPointer() noexcept : d(0) {}
+    Q_NODISCARD_CTOR constexpr QTaggedPointer(std::nullptr_t) noexcept : QTaggedPointer() {}
 
-    explicit QTaggedPointer(T *pointer, Tag tag = Tag()) noexcept
+    Q_NODISCARD_CTOR explicit QTaggedPointer(T *pointer, Tag tag = Tag()) noexcept
         : d(quintptr(pointer) | quintptr(tag))
     {
         static_assert(sizeof(Type*) == sizeof(QTaggedPointer));
@@ -72,12 +72,31 @@ 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
     {
-        d = reinterpret_cast<quintptr>(other) | (d & tagMask());
+        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>(static_cast<T *>(nullptr)) | (d & tagMask());
+        return *this;
+    }
+#endif
+
     static constexpr Tag maximumTag() noexcept
     {
         return TagType(typename QtPrivate::TagInfo<T>::TagType(tagMask()));
@@ -85,8 +104,10 @@ public:
 
     void setTag(Tag tag)
     {
-        Q_ASSERT_X((static_cast<typename QtPrivate::TagInfo<T>::TagType>(tag) & pointerMask()) == 0,
-            "QTaggedPointer<T, Tag>::setTag", "Tag is larger than allowed by number of available tag bits");
+        Q_ASSERT_X(
+                (static_cast<quintptr>(tag) & pointerMask()) == 0,
+                "QTaggedPointer<T, Tag>::setTag",
+                "Tag is larger than allowed by number of available tag bits");
 
         d = (d & pointerMask()) | static_cast<quintptr>(tag);
     }