polymorphic variant typing problem

bucklescript

   17 ┆ jsStuff =>
   18 ┆   switch (beeTypeFromJs(jsStuff)) {
  >19 ┆   | Some(output) => output
   20 ┆   | None => `baa
   21 ┆   };
  
  This has type:
    beeType
  But somewhere wanted:
    stuff4
  The first variant type does not allow tag(s)
  `caa, `cii, `coo, `cuu, `daa, `fii

stuff.re

[@bs.deriving jsConverter]
type beeType = [ | `baa | `bii | `boo | `buu];

[@bs.deriving jsConverter]
type ceeType = [ | `caa | `cii | `coo | `cuu];

[@bs.deriving jsConverter]
type deeType = [ | `daa];

[@bs.deriving jsConverter]
type feeType = [ | `fii];

type stuff4 = [ beeType | ceeType | deeType | feeType];
type stuff2 = [ beeType | ceeType];

let simplifiedFromJs: string => [ stuff4] =
  jsStuff =>
    switch (beeTypeFromJs(jsStuff)) {
    | Some(output) => output
    | None => `baa
    };

/* stuff2FromJs and stuff4FromJs are what I really want to do */

let stuff2FromJs: string => [ stuff2] =
  jsStuff =>
    switch (beeTypeFromJs(jsStuff)) {
    | Some(output) => output
    | None =>
      switch (ceeTypeFromJs(jsStuff)) {
      | Some(output) => output
      | None => `baa
      }
    };

let stuff4FromJs: string => [ stuff4] =
  jsStuff =>
    switch (beeTypeFromJs(jsStuff)) {
    | Some(output) => output
    | None =>
      switch (ceeTypeFromJs(jsStuff)) {
      | Some(output) => output
      | None =>
        switch (deeTypeFromJs(jsStuff)) {
        | Some(output) => output
        | None =>
          switch (feeTypeFromJs(jsStuff)) {
          | Some(output) => output
          | None => `baa
          }
        }
      }
    };

So… the correct code is

let simplifiedFromJs: string => [ stuff4] =
  jsStuff =>
    switch (beeTypeFromJs(jsStuff)) {
    | Some(output) => (output :> stuff4)
    | None => `baa
    };