C#: Add compound assignment operator data flow example.

Author: michaelnebel

csharp/ql/test/library-tests/dataflow/operators/Operator.cs

@@ -84,4 +84,39 @@ public void M6()
         var y = Source<C>(12);
         M6Aux(x, y);
     }
-}
+}
+
+public class CompoundAssignmentOperators
+{
+    static void Sink(object o) { }
+    static T Source<T>(object source) => throw null;
+
+    public class C
+    {
+        public object Field { get; private set; }
+
+        public C()
+        {
+            Field = new object();
+        }
+
+        public C(object o)
+        {
+            Field = o;
+        }
+
+        public void operator +=(C x)
+        {
+            Field = x.Field;
+        }
+    }
+
+    public void M1()
+    {
+        var tainted = Source<object>(1);
+        var x = new C();
+        var y = new C(tainted);
+        x += y;
+        Sink(x.Field); // $ hasValueFlow=1
+    }
+}

csharp/ql/test/library-tests/dataflow/operators/operatorFlow.expected

@@ -219,6 +219,7 @@ subpaths
 | Operator.cs:77:29:77:29 | access to parameter y : C | Operator.cs:22:51:22:51 | y : C | Operator.cs:22:57:22:57 | access to parameter y : C | Operator.cs:77:25:77:29 | call to operator checked / : C |
 | Operator.cs:77:29:77:29 | access to parameter y : C | Operator.cs:22:51:22:51 | y : C | Operator.cs:22:57:22:57 | access to parameter y : C | Operator.cs:77:25:77:29 | call to operator checked / : C |
 testFailures
+| Operator.cs:120:24:120:42 | // ... | Missing result: hasValueFlow=1 |
 #select
 | Operator.cs:30:14:30:14 | access to local variable z | Operator.cs:27:17:27:28 | call to method Source<C> : C | Operator.cs:30:14:30:14 | access to local variable z | $@ | Operator.cs:27:17:27:28 | call to method Source<C> : C | call to method Source<C> : C |
 | Operator.cs:30:14:30:14 | access to local variable z | Operator.cs:27:17:27:28 | call to method Source<C> : C | Operator.cs:30:14:30:14 | access to local variable z | $@ | Operator.cs:27:17:27:28 | call to method Source<C> : C | call to method Source<C> : C |