Add a very basic guard against virtual fields which reference themself

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -617,6 +617,15 @@ void QgsVectorLayerFeatureIterator::prepareExpression( int fieldIdx )
   exp->setAreaUnits( QgsProject::instance()->areaUnits() );
 
   exp->prepare( mExpressionContext.get() );
+  Q_FOREACH ( const QString &col, exp->referencedColumns() )
+  {
+    if ( mSource->fields().lookupField( col ) == fieldIdx )
+    {
+      // circular reference - expression depends on column itself
+      delete exp;
+      return;
+    }
+  }
   mExpressionFieldInfo.insert( fieldIdx, exp );
 
   Q_FOREACH ( const QString &col, exp->referencedColumns() )

tests/src/python/test_qgsvectorlayer.py

@@ -1609,6 +1609,11 @@ def test_ExpressionField(self):
 
         self.assertEqual(layer.pendingFields().count(), cnt)
 
+        # expression field which references itself
+        idx = layer.addExpressionField('sum(test2)', QgsField('test2', QVariant.LongLong))
+        fet = next(layer.getFeatures())
+        self.assertEqual(fet['test2'], NULL)
+
     def test_ExpressionFieldEllipsoidLengthCalculation(self):
         #create a temporary layer
         temp_layer = QgsVectorLayer("LineString?crs=epsg:3111&field=pk:int", "vl", "memory")