C++: Demonstrate FP in AV Rule 82

The added test is a reduced version of a FP observed in the wild.

cpp/ql/test/query-tests/jsf/4.10 Classes/AV Rule 82/AV Rule 82.cpp

@@ -181,11 +181,35 @@ private:
   Forgivable operator=(int *_val);
 };
 
+// This template has structure similar to `std::enable_if`.
+template<typename S, typename T>
+struct second {
+  typedef T type;
+};
+
+struct TemplatedAssignmentGood {
+  template<typename T>
+  typename second<T, TemplatedAssignmentGood &>::type operator=(T val) { // GOOD [FALSE POSITIVE]
+    return *this;
+  }
+};
+
+struct TemplatedAssignmentBad {
+  template<typename T>
+  typename second<T, TemplatedAssignmentBad>::type operator=(T val) { // BAD (missing &)
+    return *this;
+  }
+};
+
 int main() {
   Container c;
   c = c;
   TemplateReturnAssignment<int> tra(1);
   tra = 2;
   tra = true;
+  TemplatedAssignmentGood taGood;
+  taGood = 3;
+  TemplatedAssignmentBad taBad;
+  taBad = 4;
   return 0;
 }

cpp/ql/test/query-tests/jsf/4.10 Classes/AV Rule 82/AV Rule 82.expected

@@ -2,3 +2,6 @@
 | AV Rule 82.cpp:24:8:24:16 | operator= | Assignment operator in class Bad2 should have return type Bad2&. Otherwise a copy is created at each call. |
 | AV Rule 82.cpp:63:29:63:29 | operator= | Assignment operator in class TemplateReturnAssignment<int> does not return a reference to *this. |
 | AV Rule 82.cpp:63:29:63:37 | operator= | Assignment operator in class TemplateReturnAssignment<T> does not return a reference to *this. |
+| AV Rule 82.cpp:192:55:192:63 | operator= | Assignment operator in class TemplatedAssignmentGood does not return a reference to *this. |
+| AV Rule 82.cpp:199:52:199:52 | operator= | Assignment operator in class TemplatedAssignmentBad does not return a reference to *this. |
+| AV Rule 82.cpp:199:52:199:60 | operator= | Assignment operator in class TemplatedAssignmentBad does not return a reference to *this. |