Superfluous need for a custom constructor

[vinipsmaker]

So I have this code (I'm not gonna try to shorten even more because the code is already pretty short):

class Token(nud_impl, led_impl, lbp):
    nonfinal
    constructor(~nud: nud = #false, ~led: led = #false, ~lbp: lbp = 0):
        super(nud, led, lbp)
    method nud(): nud_impl(this)
    method led(lhs): led_impl(this, lhs)

class RParen():
    extends Token
    constructor(): super()()

When I try to instantiate a RParen, it works:

let t = RParen()

However if I remove the constructor line from RParen:

constructor(): super()()

Racket complains quite a lot. What's the value of this line anyways? It's doing nothing here.

[usaoc]

We don’t do anything to guess how the custom constructor for the superclass should be called, so it would be called like the default constructor (with three by-position arguments for the fields), which causes a run-time arity error. (I think this should be improved as a compile-time error, but more work needs to be done, cf. #459.) But, the code doesn’t need a custom constructor anyway, if you just move the keywords into the field specifications:

class Token(~nud: nud_impl = #false,
            ~led: led_impl = #false,
            ~lbp: lbp = 0):
  nonfinal
  method nud(): nud_impl(this)
  method led(lhs): led_impl(this, lhs)

class RParen():
  extends Token