QHash/QSet: fix squeeze() for default-constructed container

QHash::squeeze() was unconditionally calling reserve(0), which is
always allocating memory (even for 0 size).
This was leading to a confusing situation when calling squeeze() on
a default-constructed container with 0 capacity() actually allocated
memory. This is very misleading, as squeeze() is supposed to free
unneeded memory, not to allocate more.

This patch adds a check for non-zero capacity. As a result, nothing
is done for default-constructed container.

Note that this patch also affects the QSet::squeeze() behavior, because
QSet uses QHash as its underlying data type.

Task-number: QTBUG-91736
Change-Id: Ib1c3c8b7b3de6ddeefea0e70b1ec71803e8fd3b3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andreas Buhr <andreas.buhr@qt.io>
(cherry picked from commit b095d268788343b67a3995db7148dcc3af9bde1a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qhash/tst_qhash.cpp b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
index 0ca37b5f7a..3c873d093e 100644
--- a/tests/auto/corelib/tools/qhash/tst_qhash.cpp
+++ b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
@@ -2587,6 +2587,8 @@ void tst_QHash::fineTuningInEmptyHash()
 {
     QHash<QString, int> hash;
     QCOMPARE(hash.capacity(), 0);
+    hash.squeeze();
+    QCOMPARE(hash.capacity(), 0);
     QVERIFY(qFuzzyIsNull(hash.load_factor()));
     QVERIFY(!hash.isDetached());