FYI here's a variant that eliminates the `impossible` function and instead uses an assignment to the same effect:

https://www.typescriptlang.org/play/#src=const%20enum%20Scre...

This is what uglify makes of that:

    var needsCancelButton = function (e) {
      switch (e) {
        case 0:
        case 1:
          return!0;
        case 2:
          return!1
      }
      return e
    };

EDIT: Just using `return screen` without the reassignment also works because the return type of the function and the type of the argument are incompatible, but that seems a lot more error prone and less widely applicable that explicitly reassigning to a `never` variable and returning that.