Type "{ a: boolean }" is incompatible with type "{ a: false } | { a: true }"

[thecotne]

type Data = { a: true } | { a: false }

function thisDoesNot(data: Data): Data {
  return {
    a: data.a
  }
}

try

[thecotne]

message has changed from Type "{ a: false | true }" is incompatible with type "{ a: false } | { a: true }" to Type "{ a: boolean }" is incompatible with type "{ a: false } | { a: true }"

i think change was introduced with this commit https://github.com/JSMonk/hegel/commit/95fdae84819b1f131110456f7e1a1f7aa2d39da1

[JSMonk]

Yes. And I don't know how to fix it for now :( I'm about the issue. The issue exists in Flow Example. And doesn't exist in TypeScript Example. But with TypeScript, there is problem, because of type { a: false | true } equal to type { a: false } | { a: true }. And we easily can get the next error:

function process(a: { value: true } | { value: false }) {
  a.value = false;
}

const CONSTANT: { value: true } = { value: true };

process(CONSTANT);

Try

[thecotne]

@JSMonk that's how typescript's type assignment works there is no safety or soundness in it see https://github.com/microsoft/TypeScript/issues/33748

but i understand why it's complicated to implement

i think this should work as well (does not work in ts and flow)

type Data = { a: string } | { a: number }

function thisDoesNot(data: Data): Data {
  return {
    a: data.a
  }
}

try

editing of union object is different story for that you need to refine type to sub set of union objects that allow type of modification developer is intended to do for example

type Data = { a: string } | { a: number }

function notOk(data: Data) {
  data.a = 'sdsds'//err
}

function ok(data: Data) {
  if (typeof data.a === 'string') {
    data.a = 'sds'//ok
  }
  if (typeof data.a === 'number') {
    data.a = 111//ok
  }
}