How to report a bug about MonkeyC?

Hi, sir/mam

      I have the follwing code in monkeyC 

import Toybox.Graphics;
import Toybox.Lang;
import Toybox.Math;
import Toybox.System;
//! Initial app settings view
class ToolKit {

public function foo() as Void
{


var tmp = Abs(-1); // this line will cause runtime error that the Abs function can not be found.

tmp = self.Abs(-1); // this line will cause runtime error that the Abs function can not be found.

tmp = ToolKit.Abs(-1);// This line will work. 


}

public function Abs( x as Float or Number) as Float or Number {
if (x < 0) {
return -x;
} else {
return x;
}
}


}

It will generate a runtime error like: Abs is not found for the directly invoking or with self.Abs. Why? 

Thank you for your time.