C#: Make postupdate notes for conditional branches.

Author: michaelnebel

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

@@ -177,6 +177,32 @@ predicate hasNodePath(ControlFlowReachabilityConfiguration conf, ExprNode n1, No
   )
 }
 
+/**
+ * Gets a node that may execute last in `n`, and which, when it executes last,
+ * will be the value of `n`.
+ */
+private ControlFlow::Nodes::ExprNode getALastEvalNode(ControlFlow::Nodes::ExprNode cfn) {
+  exists(ConditionalExpr e | cfn.getExpr() = e | result.getExpr() = [e.getThen(), e.getElse()])
+}
+
+private predicate relevantArgumentType(ControlFlow::Nodes::ExprNode cfn) {
+  exists(Expr e | cfn.getExpr() = e |
+    exists(Type t | t = e.stripCasts().getType() |
+      t instanceof RefType and
+      not t instanceof NullType
+      or
+      t = any(TypeParameter tp | not tp.isValueType())
+    )
+  )
+}
+
+/** Gets a node for which to construct a post-update node for argument `arg`. */
+private ControlFlow::Nodes::ExprNode getAPostUpdateNodeForArg(Argument arg) {
+  result = getALastEvalNode*(arg.getAControlFlowNode()) and
+  relevantArgumentType(result) and
+  not exists(getALastEvalNode(result))
+}
+
 /** Provides predicates related to local data flow. */
 module LocalFlow {
   private class LocalExprStepConfiguration extends ControlFlowReachabilityConfiguration {
@@ -719,14 +745,9 @@ private module Cached {
       cfn.getElement().(ObjectCreation).hasInitializer()
     } or
     TExprPostUpdateNode(ControlFlow::Nodes::ExprNode cfn) {
+      cfn = getAPostUpdateNodeForArg(_)
+      or
       exists(Expr e | e = cfn.getExpr() |
-        exists(Type t | t = e.(Argument).stripCasts().getType() |
-          t instanceof RefType and
-          not t instanceof NullType
-          or
-          t = any(TypeParameter tp | not tp.isValueType())
-        )
-        or
         fieldOrPropertyStore(_, _, _, e, true)
         or
         arrayStore(_, _, e, true)
@@ -1921,7 +1942,18 @@ private module PostUpdateNodes {
 
     ExprPostUpdateNode() { this = TExprPostUpdateNode(cfn) }
 
-    override ExprNode getPreUpdateNode() { cfn = result.getControlFlowNode() }
+    override ExprNode getPreUpdateNode() {
+      // For compund arguments, such as `m(if b then x else y)`, we want the leaf nodes
+      // `[post] x` and `[post] y` to have two pre-update nodes: (1) the compund argument,
+      // `if b then x else y`; and the (2) the underlying expressions; `x` and `y`,
+      // respectively.
+      //
+      // This ensures that we get flow out of the call into both leafs (1), while still
+      // maintaining the invariant that the underlying expression is a pre-update node (2).
+      cfn = getAPostUpdateNodeForArg(result.asExpr())
+      or
+      cfn = result.getControlFlowNode()
+    }
 
     override DataFlowCallable getEnclosingCallableImpl() {
       result.asCallable() = cfn.getEnclosingCallable()