ReScript v12 - pattern matching on optional fields breaks other cases

[DZakh]

https://rescript-lang.org/try?version=v12.0.0-alpha.10&module=esmodule&code=C4TwDgpgBMCGDmUC8UBCB7dAbCsB2UAPlAHICuAtgEYQBOAUKJFAGbq0WzDJQCSewAMwAmeo3DQAzgGMAFhE48A3vShRp6PJOAB+AFxQqmLABpVrdp10G2HLmbVx4Bp2YC+YnN1oQWASzxoFBl5RSQAPigVNUkAdz9gOSgQhVgo82IlDS1gAwB9N2RIgCJijKjbKwN+IWFCiKhigNqytUynAwxsXDx6kqNu-FaiKDyixoATX1gyLGBhjw8gA

v12 (broken):

function refine(schema) {
  if (schema.const !== undefined) {
    return "";
  } else {
    return "int32";
  }
}

v11 (working):

function refine(schema) {
  if (schema.const !== undefined) {
    return "";
  } else if (schema.format !== undefined) {
    return "int32";
  } else if (schema.tag === "Boolean") {
    return "boolean";
  } else {
    return "default";
  }
}

[DZakh]

Broken between alpha.4 and alpha.5

[cristianoc]

This seems to happen when variant format has only one case, and that single case Int32 is used as a case in the pattern matching.

type tag = Boolean | Number
type format = Int32

type schema = {
  const?: bool,
  format?: format,
  tag: tag,
}

let refine = schema => {
  switch schema {
  | {const: _} => ""
  | {format: Int32} => "int32"
  | {tag: Boolean} => "boolean"
  | _ => "default"
  }
}

[cristianoc]

Here's a simplified example, showing that using the explicit single value triggers the issue:

type format = Int32

type schema = {
  format?: format,
}

let bad = schema => {
  switch schema {
  | {format: Int32} => "int32"
  | _ => "default"
  }
}

let good = schema => {
  switch schema {
  | {format: _} => "int32"
  | _ => "default"
  }
}

compiles to:

function bad(schema) {
  return "int32";
}

function good(schema) {
  if (schema.format !== undefined) {
    return "int32";
  } else {
    return "default";
  }
}