Flow: Misguiding error - object literal incompatible with object type

[R0bson]

Flow version: 0.38

Received error:
Flow: object literal. This type is incompatible with object type

Expected error
Flow: undefined. This type is incompatible with number

How to reproduce:

type PhotoDetailsType = {
  path: string,
  width: number,
  height: number,
  size: number, // here should be ?number
}

type Photo = {
  mq: PhotoDetailsType,
}

const photo: Photo = {
  mq: {
    path: '/static/images/profilePhotos/6596/696840_mq.jpg',
    width: 920,
    height: 1636,
    size: undefined, // this causes a problem
  }
}

[jesseschalken]

It gave the correct error in 0.37.4 (Try Flow - toggle the version in the top right):

18:     size: undefined, // this causes a problem
              ^ undefined. This type is incompatible with
6:   size: number, // here should be ?number
           ^ number

[R0bson]

One more example (minimalistic):
Received error (v0.38.0):
Flow: object literal. This type is incompatible with object type

Expected error (v0.37.4):
Flow: undefined. This type is incompatible with the expected return type of string

Try Flow

[mhaddon]

I can confirm this bug, it is annoying because it sends you down the wrong rabbit hole

[madorb]

Seeing the same issue in flow 0.45.0

[sabbyt]

And seeing the same in 0.47.0

[rsolomon]

Still an issue with 0.50.0 ;)

[insidewhy]

Also seems misleading when you try parameter destructuring.

function blah({ thing = null }: { thing: Thing | null })

Tells me null is incompatible with object type and also that object type is incompatible with null. Maybe this is another bug.

[samwgoldman]

I'm not sure what the "still an issue" comments are referring to, but the original example is resolved.

Note that the issue was just that Try Flow didn't display the entire error message. It now matches the Flow CLI, which outputs:

Error: test.js:14
                          v
 14: const photo: Photo = {
 15:   mq: {
 16:     path: '/static/images/profilePhotos/6596/696840_mq.jpg',
...:
 21: }
     ^ object literal. This type is incompatible with
 14: const photo: Photo = {
                  ^^^^^ object type
  Property `mq` is incompatible:
               v
     15:   mq: {
     16:     path: '/static/images/profilePhotos/6596/696840_mq.jpg',
     17:     width: 920,
    ...:
     20:   }
           ^ object literal. This type is incompatible with
     11:   mq: PhotoDetailsType,
               ^^^^^^^^^^^^^^^^ object type
      Property `size` is incompatible:
         19:     size: undefined, // this causes a problem
                       ^^^^^^^^^ undefined. This type is incompatible with
          7:   size: number, // here should be ?number
                     ^^^^^^ number


Found 1 error

[madorb]

@samwgoldman I was getting this error from the CLI not from Try Flow.

[samwgoldman]

@madorb OK, sorry about that. In any event, both output the same information now, which is enough information to be useful, I think.