3 files changed, 12 insertions, 9 deletions
diff --git a/src/corelib/tools/qduplicatetracker_p.h b/src/corelib/tools/qduplicatetracker_p.h
index 99068c01a3..68284fb916 100644
--- a/src/corelib/tools/qduplicatetracker_p.h
+++ b/src/corelib/tools/qduplicatetracker_p.h
@@ -64,11 +64,18 @@ QT_BEGIN_NAMESPACE
 template <typename T, size_t Prealloc = 32>
 class QDuplicateTracker {
 #ifdef __cpp_lib_memory_resource
-    char buffer[Prealloc * sizeof(T)];
+    struct node_guesstimate { void *next; size_t hash; T value; };
+    static constexpr size_t bufferSize(size_t N) {
+        return N * sizeof(void*) // bucket list
+                + N * sizeof(node_guesstimate); // nodes
+    }
+
+    char buffer[bufferSize(Prealloc)];
     std::pmr::monotonic_buffer_resource res{buffer, sizeof buffer};
-    std::pmr::unordered_set<T> set{&res};
+    std::pmr::unordered_set<T> set{Prealloc, &res};
 #else
-    QSet<T> set;
+    static QSet<T> makeQSet() { QSet<T> r; r.reserve(Prealloc); return r; }
+    QSet<T> set = makeQSet();
     int setSize = 0;
 #endif
     Q_DISABLE_COPY_MOVE(QDuplicateTracker);
diff --git a/src/corelib/tools/qsharedpointer_impl.h b/src/corelib/tools/qsharedpointer_impl.h
index 790c187cb9..8abb57586b 100644
--- a/src/corelib/tools/qsharedpointer_impl.h
+++ b/src/corelib/tools/qsharedpointer_impl.h
@@ -1012,15 +1012,11 @@ std::shared_ptr<X> qobject_pointer_cast(std::shared_ptr<T> &&src)
     using element_type = typename std::shared_ptr<X>::element_type;
     auto castResult = qobject_cast<element_type *>(src.get());
     if (castResult) {
-        auto result = std::shared_ptr<X>(std::move(src), castResult);
-#if __cplusplus <= 201703L
         // C++2a's move aliasing constructor will leave src empty.
         // Before C++2a we don't really know if the compiler has support for it.
         // The move aliasing constructor is the resolution for LWG2996,
         // which does not impose a feature-testing macro. So: clear src.
-        src.reset();
-#endif
-        return result;
+        return std::shared_ptr<X>(qExchange(src, nullptr), castResult);
     }
     return std::shared_ptr<X>();
 }
diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
index fd0c99c032..0110956b77 100644
--- a/src/corelib/tools/qvarlengtharray.h
+++ b/src/corelib/tools/qvarlengtharray.h
@@ -507,8 +507,8 @@ Q_OUTOFLINE_TEMPLATE typename QVarLengthArray<T, Prealloc>::iterator QVarLengthA
 
     int offset = int(before - ptr);
     if (n != 0) {
+        const T copy(t); // `t` could alias an element in [begin(), end()[
         resize(s + n);
-        const T copy(t);
         if (!QTypeInfoQuery<T>::isRelocatable) {
             T *b = ptr + offset;
             T *j = ptr + s;