fix FP in superfluous-trailing-arguments related to Function.arguments

Author: erik-krogh

javascript/ql/src/semmle/javascript/Functions.qll

@@ -117,7 +117,13 @@ class Function extends @function, Parameterized, TypeParameterized, StmtContaine
   ArgumentsVariable getArgumentsVariable() { result.getFunction() = this }
 
   /** Holds if the body of this function refers to the function's `arguments` variable. */
-  predicate usesArgumentsObject() { exists(getArgumentsVariable().getAnAccess()) }
+  predicate usesArgumentsObject() {
+    exists(getArgumentsVariable().getAnAccess()) or
+    exists(PropAccess read | 
+      read.getBase() = getVariable().getAnAccess() and
+      read.getPropertyName() = "arguments"
+    )
+  }
 
   /**
    * Holds if this function declares a parameter or local variable named `arguments`.

javascript/ql/test/query-tests/LanguageFeatures/SpuriousArguments/tst.js

@@ -120,3 +120,13 @@ parseFloat("123", 10);
 	throwerWithParam(42, 87); // NOT OK
 	throwerIndirect(42); // OK, but still flagged due to complexity
 });
+
+function sum2() {
+	var result = 0;
+	for (var i=0,n=sum2.arguments.length; i<n; ++i)
+		result += sum2.arguments[i];
+	return result;
+}
+
+// OK
+sum2(1, 2, 3);