C++: fix join order in IR virtual dispatch

Author: rdmarsh2

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowDispatch.qll

@@ -70,8 +70,7 @@ private module VirtualDispatch {
         // Call return
         exists(DataFlowCall call, ReturnKind returnKind |
           other = getAnOutNode(call, returnKind) and
-          src.(ReturnNode).getKind() = returnKind and
-          call.getStaticCallTarget() = src.getEnclosingCallable()
+          returnNodeWithKindAndEnclosingCallable(src, returnKind, call.getStaticCallTarget())
         ) and
         allowFromArg = false
         or
@@ -125,6 +124,16 @@ private module VirtualDispatch {
     }
   }
 
+  /**
+   * A ReturnNode with its ReturnKind and its enclosing callable.
+   * 
+   * Used to fix a join ordering issue in flowsFrom.
+   */
+  private predicate returnNodeWithKindAndEnclosingCallable(ReturnNode node, ReturnKind kind, DataFlowCallable callable) {
+    node.getKind() = kind and
+    node.getEnclosingCallable() = callable
+  }
+
   /** Call through a function pointer. */
   private class DataSensitiveExprCall extends DataSensitiveCall {
     DataSensitiveExprCall() { not exists(this.getStaticCallTarget()) }