Fix to not report on enum member initialization

Author: tamasvajk

csharp/ql/src/Likely Bugs/SelfAssignment.ql

@@ -20,7 +20,9 @@ class StructuralComparisonConfig extends StructuralComparisonConfiguration {
     exists(AssignExpr ae |
       // Member initializers are never self-assignments, in particular
       // not initializers such as `new C { F = F };`
-      not ae instanceof MemberInitializer
+      not ae instanceof MemberInitializer and
+      // Enum field initializers are never self assignments. `enum E { A = 42 }`
+      not ae.getParent().(Field).getDeclaringType() instanceof Enum
     |
       ae.getLValue() = x and
       ae.getRValue() = y

csharp/ql/test/query-tests/Likely Bugs/SelfAssignment/selfassigns.cs

@@ -82,4 +82,11 @@ public void NotOK(SelfAssigns obj, int y)
         this.Self.Self.Self.StringProp = Self.Self.Self.StringProp;
         intArray[1] = this.intArray[1 + 0];
     }
+
+    enum Enum
+    {
+        X = 42,
+        Y = 100,
+        Z
+    }
 }