Fix less-than comparison for QStandardItem and QSortFilterProxyModel with invalid data.

Previously, QStandardItem::operator<() returned true when both
items had invalid data. With MSVC in debug mode (checked
iterators/STL), this triggered an assert in
tst_QStandardItem::sortChildren() since that verifies
that !(b < a) when a < b:

Debug Assertion Failed!
Line: 3006
Expression: invalid operator<

Introduce a stable sort order for invalid items such that
other items are always less than invalid items and comparing
invalid items returns false (indicating equivalence).

Change-Id: Ica0f0d9f001c86973b1941dbcc1faf282e4c47df
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/corelib/itemmodels/qsortfilterproxymodel.cpp b/src/corelib/itemmodels/qsortfilterproxymodel.cpp
index 922d0f1622..38f8dfa788 100644
--- a/src/corelib/itemmodels/qsortfilterproxymodel.cpp
+++ b/src/corelib/itemmodels/qsortfilterproxymodel.cpp
@@ -2617,9 +2617,12 @@ bool QSortFilterProxyModel::lessThan(const QModelIndex &source_left, const QMode
     Q_D(const QSortFilterProxyModel);
     QVariant l = (source_left.model() ? source_left.model()->data(source_left, d->sort_role) : QVariant());
     QVariant r = (source_right.model() ? source_right.model()->data(source_right, d->sort_role) : QVariant());
+    // Duplicated in QStandardItem::operator<()
+    if (l.userType() == QVariant::Invalid)
+        return false;
+    if (r.userType() == QVariant::Invalid)
+        return true;
     switch (l.userType()) {
-    case QVariant::Invalid:
-        return (r.type() != QVariant::Invalid);
     case QVariant::Int:
         return l.toInt() < r.toInt();
     case QVariant::UInt: