C#: Take compound assignment operator argument shift into account for operators declared as extensions.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/exprs/Call.qll

@@ -525,6 +525,16 @@ class ExtensionOperatorCall extends OperatorCall {
   ExtensionOperatorCall() { this.getTarget() instanceof ExtensionOperator }
 
   override string getAPrimaryQlClass() { result = "ExtensionOperatorCall" }
+
+  private predicate isOrdinaryStaticCall() {
+    not exists(Expr e | e = this.getChildExpr(-1) | not e instanceof TypeAccess)
+  }
+
+  override Expr getArgument(int i) {
+    exists(int j | result = this.getChildExpr(j) |
+      if this.isOrdinaryStaticCall() then (j = i and j >= 0) else (j = i - 1 and j >= -1)
+    )
+  }
 }
 
 /**