Default argument in recursive functions crashes compiler

[marcusroberts]

When I set a default value for an argument in recursive functions, the compiler crashes with

Failure("Impossible: optional argument with non-option type")

let rec parenthesizedExpression = () => {
  let expression1 = expression(0)
  expression1
}

and expression = (prec=0) => {  // setting the default value here causes the compiler crash
  0
}

provide let parse = (source: String) => {
  expression(0)
}

[spotandjake]

This is a really interesting error, it occurs when Apply_too_many_arguments is hit in typecore.re, interestingly if you pass the arguments as named parameters we get a different impossible error. This error also only seems to occur when the function being used is called before its defintion in an and statement.

In grain we require arguments to be passed with their label in the defaulted case normally we have a pretty error for this but it seems something is messing it up. I traced this issue back a little and we seem to be producing the wrong type here let funct = type_exp(env, func); line 1382 in type_expect we are producing a function with a (Default(_), TTyVar(None)) instead of an optional type.