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>

3 files changed, 11 insertions, 5 deletions

diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 624d647c72..25c00ce26f 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -850,7 +850,11 @@ public:
         else
             d = Data::detached(d, size_t(size));
     }
-    inline void squeeze() { reserve(0); }
+    inline void squeeze()
+    {
+        if (capacity())
+            reserve(0);
+    }
 
     inline void detach() { if (!d || d->ref.isShared()) d = Data::detached(d); }
     inline bool isDetached() const noexcept { return d && !d->ref.isShared(); }
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());
 
diff --git a/tests/auto/corelib/tools/qset/tst_qset.cpp b/tests/auto/corelib/tools/qset/tst_qset.cpp
index 2911f822e0..490e8e97fd 100644
--- a/tests/auto/corelib/tools/qset/tst_qset.cpp
+++ b/tests/auto/corelib/tools/qset/tst_qset.cpp
@@ -244,12 +244,12 @@ void tst_QSet::reserve()
 void tst_QSet::squeeze()
 {
     QSet<int> set;
-    int n = set.capacity();
-    QVERIFY(n == 0);
+    QCOMPARE(set.capacity(), 0);
 
     set.squeeze();
-    n = set.capacity();
-    QVERIFY(n > 0 && n < 100);
+    QCOMPARE(set.capacity(), 0);
+
+    QVERIFY(!set.isDetached());
 
     set.reserve(1000);
     QVERIFY(set.capacity() >= 1000);