ReScript 9.1: weird error message if optional prop has no type annotation

[cknitt]

Consider the following React component:

Test.resi:

@react.component
let make: (~s: string=?) => React.element

Test.res:

@react.component
let make = (~s=?) =>
  switch s {
  | Some(s) => React.string(s)
  | None => React.null
  }

This compiles fine in ReScript 9.0.

In ReScript 9.1, it gives me the following weird error message:

  The implementation /Users/christoph/projects/cca/xxx/src/navigation/Test.res
       does not match the interface src/navigation/test.cmi:
       Values do not match:
         external makeProps: (~?s: 's, ~?key: string, unit) => {"s": option<'s>} =
  "" "
      A!s@A#keyA@@@"
       is not included in
         external makeProps: (~?s: string, ~?key: string, unit) => {
  "s": option<string>,
} =
  "" "
      A!sAA#keyA@@@"
       File "/Users/christoph/projects/cca/xxx/src/navigation/Test.resi", line 1, characters 0-58:
         Expected declaration
       File "Test", line 1: Actual declaration

If I change Test.res to

@react.component
let make = (~s: option<string>=?) =>
  switch s {
  | Some(s) => React.string(s)
  | None => React.null
  }

it compiles fine with ReScript 9.1.

[bobzhang]

this maybe related to jsx cc @IwanKaramazow @rickyvetter

[anmonteiro]

This was introduced in #4960.

It only happens if there's an interface file. ReScript started trying to unbox options by checking whether it has been inferred to option<_>, but in this case the inference just says 'a.

[rickyvetter]

Definitely has to do with the obj tags that ReScript React generates under the hood. I can reproduce with this code:

module Test: {
  @obj
  external makeProps: (~s: string=?, unit) => Js.t<{"s": string}> = ""
} = {
  @obj
  external makeProps: (~s: 's=?, unit) => Js.t<{"s": 's}> = ""
}

dsource with 9.0.2:

[@@@config { flags = [|"-dsource"|] }]
module Test :
  sig
    external makeProps :
      ?s:((string)[@ns.namedArgLoc ]) -> unit -> < s: string   >  Js.t Js.t =
        ""
        "\132\149\166\190\000\000\000\012\000\000\000\007\000\000\000\018\000\000\000\018\145\160\160A\145!s\160\160@@@"
  end =
  struct
    external makeProps :
      ?s:(('s)[@ns.namedArgLoc ]) -> unit -> < s: 's   >  Js.t Js.t = ""
        "\132\149\166\190\000\000\000\012\000\000\000\007\000\000\000\018\000\000\000\018\145\160\160A\145!s\160\160@@@"
  end

dsource with e3371bd (note the differences in the external string - before they were identical):

[@@@config { flags = [|"-dsource"|] }]
module Test :
  sig
    external makeProps :
      ?s:((string)[@ns.namedArgLoc ]) -> unit -> < s: string   >  Js.t = ""
        "\132\149\166\190\000\000\000\r\000\000\000\007\000\000\000\019\000\000\000\019\145\160\160A\161!sA\160\160@@@"
  end =
  struct
    external makeProps :
      ?s:(('s)[@ns.namedArgLoc ]) -> unit -> < s: 's   >  Js.t = ""
        "\132\149\166\190\000\000\000\r\000\000\000\007\000\000\000\019\000\000\000\019\145\160\160A\161!s@\160\160@@@"
  end

[bobzhang]

This is indeed fragile, we may change the encoding in the future

[rickyvetter]

Is there a good way to avoid the breakage? Happy to modify the ppx but we don't have the info in the impl ast.

[bobzhang]

@rickyvetter I am going to fix on my side, thanks for checking