E1031

The optional argument is never supplied. If the optional argument is never supplied, then it can be safely removed from the parameter list and be used purely internally as a normal variable.

Erroneous example

Note, this warning is turned off by default. To enable this warning, you have to modify the "warn-list" of moon.pkg.json:

{
  // ...
  "warn-list": "+31"
}

main.mbt:

fn f(opt~ : Int = 4) -> Int {
  ignore(opt)
  0
}

fn main {
  println(f())
  println(f())
}

Suggestion

You can remove the optional argument, and use a local variable definition instead:

fn f() -> Int {
  let opt = 4
  ignore(opt)
  0
}

fn main {
  println(f())
  println(f())
}