Union type doesn't work when containing an imported type from another module which is dependent on the current module

[jddxf]

Consider the following two modules:

/**
 * @providesModule ModuleA
 * @flow
 */

var ModuleB = require('./ModuleB');
export type A = number;

/**
 * @providesModule ModuleB
 * @flow
 */

import type { A } from './ModuleA';
module.exports = function () {
  var a: A | null = 's';
}

Flow is expected to report an error but it doesn't.It only does when I change the type annotation on a from A | null to A or when type A is declared in ModuleB.

[asolove]

I tried to narrow this down and found that the module.exports = function() but isn't necessary. Just saying let a: A|null = "a" inside that module is sufficient for the error to not show up.

But two things are necessary:

• the two modules have to have a circular dependency
• the type that ought to fail has to be a union between a type imported from the other module and some other type that should not match the value

It looks like this:

ModuleA.js:

// @flow
var ModuleB = require('./ModuleB');
export type A = number;

ModuleB.js:

// @flow
import type { A } from './ModuleA';

let wrong: A | null = "s"; // succeeds but should error
let wrong2: A | {} = "s"; // succeeds but should error

let right1: ?A = "s"; // errors as it should
let right2: A = "s" // errors as it should
let right3: Array<A> = ["s"]; // errors as it should
let right4: {a: A} = {a: "s"}; // errors as it should

That said, having researched that farm I'm not sure what Flow should do. Possibly this is just a bug, or possibly there is something about the circular requires that Flow ought to notify us it can't deal with.

[asolove]

Ah, this is a duplicate of #3249, and is also being discussed in #3522, so I think this issue can be closed. (/cc @calebmer)

[calebmer]

I’m going to close this as a duplicate, but thanks for reporting the bug! I’ll take a look into it.