qstring.h: gcc signed overflow error when using -O2

building qt-4.8.5 with gcc-4.8.2 causes the following warning

In file included from ../../include/QtCore/qstring.h:1:0,
from ../../include/QtCore/../../src/corelib/kernel/qobject.h:48,
from ../../include/QtCore/qobject.h:1,
from kernel/qsqldriver.h:45,
from kernel/qsqldriver.cpp:42:
../../include/QtCore/../../src/corelib/tools/qstring.h: In member function 'virtual QString QSqlDriver::sqlStatement(QSqlDriver::StatementType, const QString&, const QSqlRecord&, bool) const':
../../include/QtCore/../../src/corelib/tools/qstring.h:799:11: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
{ if (i >= s.d->size) s.expand; else s.detach();