Error message for optional arguments is incorrect.

[johnridesabike]

module M: {
  let f: (~a: int) => int
} = {
  let f = (~a=1) => 1
}

Error message:

Signature mismatch:
  ...
  Values do not match:
    let f: (~?a: int) => int
  is not included in
    let f: (~a: int) => int

If the user tries to use the syntax in the message, let f: (~?a: int) => int, then it does not fix the error. To make it weirder, it seems like the parser just ignores the ? in ~?a and reports it as if it was written ~a.

Edit: here's a possibly related issue:

the rescript dump command for the implementation will output the interface without “=?” part, and it will not compile.

[amiralies]

I think this is related to #447 (unreleased)

[IwanKaramazow]

Added an extra test case, this was already fixed in master.
#447 fixes the errors not being reported. (Thanks to @amiralies)