func = => yield this // compile error

cuz coffee => compile to es5 =>
BUT WHY??? WHY????

solution:

if body of arrow (=>) function contains yield operator, it must compile in classic mode:

func = (function(_this) {
  return function* () {
    return (yield _this);
  };
})(this);

If NOT contains yield operator, then compile to es5 mode:

func = () => return this

CUZ

From the viewpoint of the programmer, there is no physical difference between normal functions and arrow functions

var func = function () { }
var func2 = () => { }

/// HOW TO KNOW ON RUNTIME WHERE ARROW FUNCTION AND WHEHE NORMAL FUNCTION???

P.S. func.toString() in arrow functions return different string code, But this is not significant at all, compared to the terrible restrictions that you impose on a programmer, forbidding to write yield in arrow functions