Python: filter local flow from a node to itself

Author: yoff

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPrivate.qll

@@ -684,8 +684,13 @@ predicate summaryFlowSteps(Node nodeFrom, Node nodeTo) {
 }
 
 predicate variableCaptureFlowStep(Node nodeFrom, Node nodeTo) {
+  // Blindly applying use-use flow can result in a node that steps to itself, for
+  // example in while-loops. To uphold dataflow consistency checks, we don't want
+  // that. However, we do want to allow `[post] n` to `n` (to handle while loops), so
+  // we should only do the filtering after `IncludePostUpdateFlow` has ben applied.
   IncludePostUpdateFlow<PhaseDependentFlow<VariableCapture::valueStep/2>::step/2>::step(nodeFrom,
-    nodeTo)
+    nodeTo) and
+  nodeFrom != nodeTo
 }
 
 /** `ModuleVariable`s are accessed via jump steps at runtime. */