diff --git a/src/corelib/tools/qset.h b/src/corelib/tools/qset.h
index b75f19f242..7330b5e91c 100644
--- a/src/corelib/tools/qset.h
+++ b/src/corelib/tools/qset.h
@@ -228,10 +228,13 @@ Q_INLINE_TEMPLATE void QSet<T>::reserve(qsizetype asize) { q_hash.reserve(asize)
 template <class T>
 Q_INLINE_TEMPLATE QSet<T> &QSet<T>::unite(const QSet<T> &other)
 {
-    if (!q_hash.isSharedWith(other.q_hash)) {
-        for (const T &e : other)
-            insert(e);
-    }
+    if (q_hash.isSharedWith(other.q_hash))
+        return *this;
+    QSet<T> tmp = other;
+    if (size() < other.size())
+        swap(tmp);
+    for (const auto &e : std::as_const(tmp))
+        insert(e);
     return *this;
 }
 
diff --git a/tests/auto/corelib/tools/qset/tst_qset.cpp b/tests/auto/corelib/tools/qset/tst_qset.cpp
index 073004d279..cbe6230207 100644
--- a/tests/auto/corelib/tools/qset/tst_qset.cpp
+++ b/tests/auto/corelib/tools/qset/tst_qset.cpp
@@ -856,7 +856,7 @@ void tst_QSet::setOperationsOnEmptySet()
 
         empty.unite(nonEmpty);
         QCOMPARE(empty, nonEmpty);
-        QVERIFY(empty.isDetached());
+        QVERIFY(!empty.isDetached());
     }
 }