Type spread failed on empty variant

[cometkim]

type a
type b =
  | ...a
  | Other

https://rescript-lang.org/try?version=v12.0.0-alpha.1&code=C4TwDgpgBAhgUKSUBGUC8cpQD5QHQHxa4DywAFhAE5xA

This failed without any proper info

[glennsl]

type a is an abstract type, not an empty variant type. In OCaml, an empty variant type would be type a = |. That doesn't seem to be supported in Rescript though.

Still not a useful error message, but conceptually they are very different and a better error message should reflect that.

[cknitt]

This seems to work:

type a = ..

type b =
  | ...a
  | Other

[glennsl]

type a = .. is an extensible variant. Which is a bit odd to support as making an open type closed would make the result depend on where it's perceived from.

Seems like in this case it ignores any variant that it should know about, and is effectively considered just an empty variant when extended:

type a = ..
type a += Foo

let f = (x: a) => switch x {
  | Foo => ()
  | _ => ()
}

type b = | ...a | Bar

let g = (x: b) => switch x {
  | Foo => ()
  | Bar => ()
}

This fails in g with the error:

The constructor Foo does not belong to type b
  
  This variant pattern is expected to have type b

I would have expected Foo to be part of b here. Or, better yet, that extending extensible variants is rejected.

[zth]

Extensible variants definitely shouldn't be allowed to be spread. So that part is a bug that we'll fix.