QVector: prevent resize() from shedding capacity

...even if reserve() hasn't been called before.

[ChangeLog][QtCore][QVector] resize() will no longer shrink the
capacity. That means resize(0) now reliably preserves capacity().

Task-number: QTBUG-39293
Done-with: Robin Burchell <robin.burchell@viroteck.net>
Change-Id: Ie7e4e597126832990b6cfb83bba875c3963b143e
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvector.cpp

@@ -384,6 +384,9 @@
     initialized with a \l{default-constructed value}. If \a size is less
     than the current size, elements are removed from the end.
 
+    Since Qt 5.6, resize() doesn't shrink the capacity anymore.
+    To shed excess capacity, use squeeze().
+
     \sa size()
 */
 

src/corelib/tools/qvector.h

@@ -406,9 +406,6 @@ void QVector<T>::resize(int asize)
     if (asize > oldAlloc) { // there is not enough space
         newAlloc = asize;
         opt = QArrayData::Grow;
-    } else if (!d->capacityReserved && asize < d->size && asize < (oldAlloc >> 1)) { // we want to shrink
-        newAlloc = asize;
-        opt = QArrayData::Grow;
     } else {
         newAlloc = oldAlloc;
     }