QVarLengthArray: fix UBs in insert(it, n, v) ([basic.life], invariants)

In the same vein as e24df8bc726d12e80f3f1d14834f9305586fcc98 for
emplace(it, v) and insert(it, rv), this patch addresses the identical
issues in insert(it, n, v). The solution is unsurprisingly the same:
q_rotate() after a resize(size() + n, v).

The 6.2- code will need to look different, because resize(n, v) didn't
exist there.

Change-Id: I1ce91969abc20f2a1e5d05a8545b009a2e0994f6
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit fed5f2445480f7cf045e93761dc902f771cbf3da)
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 6 insertions, 22 deletions

diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
index d383856736..6ae3cc8308 100644
--- a/src/corelib/tools/qvarlengtharray.h
+++ b/src/corelib/tools/qvarlengtharray.h
@@ -862,28 +862,12 @@ Q_OUTOFLINE_TEMPLATE auto QVLABase<T>::insert_impl(qsizetype prealloc, void *arr
 {
     Q_ASSERT_X(isValidIterator(before), "QVarLengthArray::insert", "The specified const_iterator argument 'before' is invalid");
 
-    qsizetype offset = qsizetype(before - cbegin());
-    if (n != 0) {
-        const T copy(t); // `t` could alias an element in [begin(), end()[
-        resize_impl(prealloc, array, size() + n);
-        if constexpr (!QTypeInfo<T>::isRelocatable) {
-            T *b = begin() + offset;
-            T *j = end();
-            T *i = j - n;
-            while (i != b)
-                *--j = *--i;
-            i = b + n;
-            while (i != b)
-                *--i = copy;
-        } else {
-            T *b = begin() + offset;
-            T *i = b + n;
-            memmove(static_cast<void *>(i), static_cast<const void *>(b), (size() - offset - n) * sizeof(T));
-            while (i != b)
-                new (--i) T(copy);
-        }
-    }
-    return data() + offset;
+    const qsizetype offset = qsizetype(before - cbegin());
+    resize_impl(prealloc, array, size() + n, t);
+    const auto b = begin() + offset;
+    const auto e = end();
+    QtPrivate::q_rotate(b, e - n, e);
+    return b;
 }
 
 template <class T>