Order of type declarations affects union behavior

[Gozala]

Here is the code to reproduce this issue with 0.15.0 installed as flow-bin from npm:

/* @flow */

type VirtualElement = Thunk|VirtualNode;
type Child = VirtualElement;
type Children = Array<Child>;


class Thunk {}
class VirtualNode {
  children: Child|Children;
  constructor(type, children/*:Children*/) {
    this.children = children.length === 1 ? children[0] :
                    children;
  }
}

I see following errors

/Users/gozala/Projects/reflex/src/core.js: Library type error:
/Users/gozala/Projects/reflex/src/core.js: inconsistent use of library definitions
/Users/gozala/Projects/reflex/src/core.js:5:17,28: array type
This type is incompatible with
/Users/gozala/Projects/reflex/src/core.js:3:23,39: union type

Found 1 error

[Gozala]

Turns out inlining types joined into Child union resolves the issue, here is a code that does not fail:

/* @flow */

type VirtualElement = Thunk|VirtualNode;
type Child = VirtualElement;
type Children = Array<Child>;


class Thunk {}
class VirtualNode {
  children: Children|Thunk|VirtualNode;
  constructor(type, children/*:Children*/) {
    this.children = children.length === 1 ? children[0] :
                    children;
  }
}

[samwgoldman]

Sounds like #582. Linking the issues together so we can check/close them all when a fix arrives.

[samwgoldman]

This is still failing after the fix for #582.

[samwgoldman]

OK... so I was able to come up with a simple repro here, and it looks like a bug. @bhosmer, you'll probably like this one.

Fails:

/* @flow */
type T = A|B;
class U {};
declare var children: U;
(children: T|U);
class A {};
class B {};

test.js:5
  5: (children: T|U);
      ^^^^^^^^ U. This type is incompatible with
  5: (children: T|U);
                ^ union: A | B

Passes: (I just moved A and B higher in the source)

/* @flow */
type T = A|B;
class U {};
declare var children: U;
class A {};
class B {};
(children: T|U);

Note this is totally non specific to classes. You can change A, B, and U to whatever (number, string, boolean) and get the same error.

[chrisui]

To save me creating a new issue I'm wondering if the error I'm encountering is caused by the issue seen here?

// decl
declare class Fixture {
  title: string;
  component: any;
  props?: Object;
  children?: Array<Object|string>|string; // this might just be bad syntax but breaks even with Object|string pulled out into a new type
  expect?: {[key:string]: Array<Function>} // see ps. note about this =D
}

// usage
export const a: Fixture = {
  title: 'title',
  component: 'component',
  children: 'Test Child'
};

decl/Fixture.js:9
  9:   children?: Array<Object|string>|string;
                  ^^^^^^^^^^^^^^^^^^^^ array type. This type is incompatible with
 17:   children: 'Test Child'
                 ^^^^^^^^^^^^ string

Ps. Unrelated but is {[key:string]: Array<Function>} a valid type? I'm trying to describe a map with values of the type Array<Function>.

[avikchaudhuri]

@Gozala, @samwgoldman: Both of your examples will be fixed in an upcoming release.

@chrisui: you seem to be exploiting an unrelated bug in your example, which has been fixed in an unrelated way: objects cannot be typed as class instances.

[avikchaudhuri]

@chrisui see also #1922, where I tried to fix your follow up example but ran into another bug