C#: Add default (reverse update) taint step from implicit operator calls to their arguments.

csharp/ql/lib/semmle/code/csharp/dataflow/internal/TaintTrackingPrivate.qll

@@ -109,6 +109,16 @@ private class LocalTaintExprStepConfiguration extends ControlFlowReachabilityCon
   }
 }
 
+private ControlFlow::Nodes::ExprNode getALastEvalNode(ControlFlow::Nodes::ExprNode cfn) {
+  exists(Expr e | any(LocalTaintExprStepConfiguration x).hasExprPath(_, result, e, cfn) |
+    e.(OperatorCall).getTarget() instanceof ImplicitConversionOperator // Should only be implicit operator calls.
+  )
+}
+
+private ControlFlow::Nodes::ExprNode getPostUpdateReverseStep(ControlFlow::Nodes::ExprNode e) {
+  result = getALastEvalNode(e)
+}
+
 private predicate localTaintStepCommon(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
   hasNodePath(any(LocalTaintExprStepConfiguration x), nodeFrom, nodeTo)
 }
@@ -177,6 +187,16 @@ private module Cached {
       readStep(nodeFrom, any(DataFlow::ContentSet c | c.isElement()), nodeTo)
       or
       nodeTo = nodeFrom.(DataFlow::NonLocalJumpNode).getAJumpSuccessor(false)
+      or
+      // Allow reverse update flow for implicit conversion operator calls.
+      // This is needed to support flow out of method call arguments, where an implicit conversion is applied
+      // to a call argument.
+      nodeTo.(PostUpdateNode).getPreUpdateNode().(DataFlow::ExprNode).getControlFlowNode() =
+        getPostUpdateReverseStep(nodeFrom
+              .(PostUpdateNode)
+              .getPreUpdateNode()
+              .(DataFlow::ExprNode)
+              .getControlFlowNode())
     ) and
     model = ""
     or