C++: Add model validation for constructor summary models

cpp/ql/lib/semmle/code/cpp/dataflow/ExternalFlow.qll

@@ -353,12 +353,26 @@
     )
   }
 
+  string getIncorrectConstructorSummaryOutput() {
+    exists(string namespace, string type, string name, string output |
+      type = name or
+      type = name + "<" + any(string s)
+    |
+      summaryModel(namespace, type, _, name, _, _, _, output, _, _, _) and
+      output.matches("ReturnValue%") and
+      result =
+        "Constructor model for " + namespace + "." + type +
+          " should use `Argument[this]` in the output, not `ReturnValue`."
+    )
+  }
+
   /** Holds if some row in a CSV-based flow model appears to contain typos. */
   query predicate invalidModelRow(string msg) {
     msg =
       [
         getInvalidModelSignature(), getInvalidModelInput(), getInvalidModelOutput(),
-        getInvalidModelSubtype(), getInvalidModelColumnCount(), KindVal::getInvalidModelKind()
+        getInvalidModelSubtype(), getInvalidModelColumnCount(), KindVal::getInvalidModelKind(),
+        getIncorrectConstructorSummaryOutput()
       ]
   }
 }