C++: Add test with FP in ExprHasNoEffect

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/ExprHasNoEffect.expected

@@ -29,6 +29,7 @@
 | test.cpp:25:3:25:3 | call to operator++ | This expression has no effect (because $@ has no external side effects). | test.cpp:9:14:9:23 | operator++ | operator++ |
 | test.cpp:62:5:62:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:47:14:47:22 | operator= | operator= |
 | test.cpp:65:5:65:5 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:55:7:55:7 | operator= | operator= |
+| test.cpp:91:5:91:19 | call to operator= | This expression has no effect (because $@ has no external side effects). | test.cpp:77:9:77:17 | operator= | operator= |
 | volatile.c:9:5:9:5 | c | This expression has no effect. | volatile.c:9:5:9:5 | c |  |
 | volatile.c:12:5:12:9 | access to array | This expression has no effect. | volatile.c:12:5:12:9 | access to array |  |
 | volatile.c:16:5:16:7 | * ... | This expression has no effect. | volatile.c:16:5:16:7 | * ... |  |

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/test.cpp

@@ -64,3 +64,45 @@ void testFunc2()
 	MyAssignable v1, v2;
 	v2 = v1;
 }
+
+namespace std {
+  typedef unsigned long size_t;
+}
+
+void* operator new  (std::size_t size, void* ptr) noexcept;
+
+struct Base {
+  Base() { }
+
+  Base &operator=(const Base &rhs) {
+    return *this;
+  }
+
+  virtual ~Base() { }
+};
+
+struct Derived : Base {
+  Derived &operator=(const Derived &rhs) {
+    if (&rhs == this) {
+      return *this;
+    }
+
+    // In case base class has data, now or in the future, copy that first.
+    Base::operator=(rhs); // GOOD [FALSE POSITIVE]
+
+    this->m_x = rhs.m_x;
+    return *this;
+  }
+
+  int m_x;
+};
+
+void use_Base() {
+  Base base_buffer[1];
+  Base *base = new(&base_buffer[0]) Base();
+
+  // In case the destructor does something, now or in the future, call it. It
+  // won't get called automatically because the object was allocated with
+  // placement new.
+  base->~Base(); // GOOD (because the call has void type)
+}