Union type passing depends on order of type definition

[willclarktech]

I'm using flow-bin 0.23.0 and getting an order effect I don't understand with a union type:

// @flow

type OptionAnswer = {
    kind           : 'OptionAnswer',
    currentAnswer? : {},
}

type TextAnswer = {
    kind           : 'TextAnswer',
    currentAnswer? : string,
}

type Answer
    = TextAnswer
    | OptionAnswer

////////////////////////////////////////////

// fine
const exampleOptionAnswer: OptionAnswer = {
    kind: 'OptionAnswer',
}

// fine
const exampleTextAnswer: TextAnswer = {
    kind: 'TextAnswer',
}

const exampleAnswer1: Answer = {
    kind: 'OptionAnswer',
}

const exampleAnswer2: Answer = {
    kind: 'TextAnswer',
}

This gives the following error:

29: const exampleAnswer1: Answer = {
                                    ^ object literal. This type is incompatible with
 29: const exampleAnswer1: Answer = {
                           ^^^^^^ union: TextAnswer | OptionAnswer
  Member 1:
   14:     = TextAnswer
             ^^^^^^^^^^ TextAnswer
  Error:
   30:     kind: 'OptionAnswer',
                 ^^^^^^^^^^^^^^ string. Expected string literal `TextAnswer`, got `OptionAnswer` instead
    9:     kind           : 'TextAnswer',
                            ^^^^^^^^^^^^ string literal `TextAnswer`
  Member 2:
   15:     | OptionAnswer
             ^^^^^^^^^^^^ OptionAnswer
  Error:
   10:     currentAnswer? : string,
                            ^^^^^^ string. This type is incompatible with
    5:     currentAnswer? : {},
                            ^^ object type

If I switch the union type definition to type Answer = OptionAnswer | TextAnswer I get this instead:

33: const exampleAnswer2: Answer = {
                                    ^ object literal. This type is incompatible with
 33: const exampleAnswer2: Answer = {
                           ^^^^^^ union: OptionAnswer | TextAnswer
  Member 1:
   14:     = OptionAnswer
             ^^^^^^^^^^^^ OptionAnswer
  Error:
   34:     kind: 'TextAnswer',
                 ^^^^^^^^^^^^ string. Expected string literal `OptionAnswer`, got `TextAnswer` instead
    4:     kind           : 'OptionAnswer',
                            ^^^^^^^^^^^^^^ string literal `OptionAnswer`
  Member 2:
   15:     | TextAnswer
             ^^^^^^^^^^ TextAnswer
  Error:
    5:     currentAnswer? : {},
                            ^^ object type. This type is incompatible with
   10:     currentAnswer? : string,
                            ^^^^^^ string

I'm pretty new to flow so maybe I'm doing something stupid, but it looks like the literal from the first disjunct is somehow infecting the object that is compared to the second disjunct?

[willclarktech]

Possibly related to #815, #582, #1455, #1194.

[mroch]

it looks like the literal from the first disjunct is somehow infecting the object that is compared to the second disjunct?

I think that's correct, and it's a bug. When an object flows to another object, optional props on the first object define "shadow" props on the second object. So in this case, exampleAnswer1 is being compared against TextAnswer first, which (incorrectly) adds a shadow prop for currentAnswer: string to exampleAnswer1, which then causes an error when it tests against OptionAnswer because string is not compatible with {}.

cc @bhosmer, @avikchaudhuri

[bhosmer]

@mroch Sounds right. It's yet another mutable effect - completely orthogonal to tvar effects - that we'll need to isolate (under any scheme for handling unions). Interestingly the simplest version of the multiverse approach involves deep-copying type terms, so it'll naturally fix this problem, but of course deep copying is also potentially a complete dealbreaker perf-wise, and avoiding it might spider the TC code with hooks in a really unpleasant way. We'll see...

[willclarktech]

It also applies when I specify a required property with an optional value in the type definition (rather than an optional property with a required value) and give a null value to the objects like this:

type TextAnswer = {
    kind          : 'TextAnswer',
    currentAnswer : ?string,
}
const exampleAnswer2: Answer = {
    kind         : 'TextAnswer',
    currentAnswer: null,
}

[avikchaudhuri]

This will be fixed in an upcoming release.

[jfrolich]

Having the same issue with a redux action union type:

export type Action =
    { type: 'FETCH_CONTENT_MESSAGES', payload: { contentItemId: number } }
  | { type: 'SIGN_IN', payload: Object, promise: PromiseType }
  | { type: 'SIGN_OUT' }
  | { type: 'JOIN_CHANNEL', payload: Object }
  | { type: 'VALIDATE_NEW_ORGANIZATION', payload: Object, promise: PromiseType }
  | { type: 'UPDATE_PROFILE', payload: Object, promise: PromiseType }
  | { type: 'RESET_PASSWORD', payload: { token: string, password: string, acceptInvite: string }}
  | { type: 'VERIFY_PASSWORD_RESET_TOKEN', payload: string }
  | { type: 'INITIALIZE_SESSION', payload: { authToken: string } }
  | { type: 'VALIDATE_NEW_USER', payload: Object, promise: PromiseType }
  | { type: 'CREATE_USER', payload: Object, promise: PromiseType }
  | { type: 'UPDATE_CONTENT_ITEM', payload: Object, promise: PromiseType }
  | { type: 'SUBSCRIBE_TO_CONTENT_ITEM', payload: { id: number } }
  | { type: 'FAVORITE_CONTENT_ITEM', payload: { id: number } }
  | { type: 'UNFAVORITE_CONTENT_ITEM', payload: { id: number } }
  | { type: 'UNSUBSCRIBE_FROM_CONTENT_ITEM', payload: { id: number } }
  | { type: 'SEND_CONTENT_MESSAGE', payload: { contentItemId: number, message: string } }
  | { type: 'CREATE_ORGANIZATION', payload: Object, promise: PromiseType }
  | { type: 'SET_MESSAGE', payload: string }

With the incorrect flow response:

  Member 18:
   25:   | { type: 'CREATE_ORGANIZATION', payload: Object, promise: PromiseType }
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ object type. See: web/static/js/actions/types.js:25
  Error:
   15:   | { type: 'VERIFY_PASSWORD_RESET_TOKEN', payload: string }
                                                           ^^^^^^ string. This type is incompatible with. See: web/static/js/actions/types.js:15
   25:   | { type: 'CREATE_ORGANIZATION', payload: Object, promise: PromiseType }
                                                   ^^^^^^ object type. See: web/static/js/actions/types.js:25

If I move CREATE_ORGANIZATION more upward in the union declaration it passes.

Is this the same bug, and if so, any projection on when this bug will be fixed?