Intersection type bug around polymorphic type aliases

[samwgoldman]

/* @flow */

type Observable<T> = { observe(observer: Observer<T>): void }
type Observer<T> = { next(x: T): void }
type Subject<T> = Observable<T> & Observer<T>

function createSubject<T>(): Subject<T> {
  var observers = []
  return {
    observe(observer) {
      observers.push(observer)
    },
    next(x) {
      for (var i = 0; i < observers.length; i++) {
        observers[i].next(x)
      }
    }
  }
}

type Test = {
  foo: Observable<string>
}

function test(): Test {
  var foo: Subject<string> = createSubject()
  return { foo }
}

test2.js|5 col 19 error|  intersection type
|| This type is incompatible with
test2.js|3 col 22 error|  object type
|| 
test2.js|5 col 19 error|  intersection type
|| This type is incompatible with
test2.js|4 col 20 error|  object type
|| 

[samwgoldman]

If I define Subject<T> without intersections, but just inlining the definitions from Observable<T> and Observer<T>, then I receive no errors.

[samwgoldman]

I was actually able to simplify the test case. The whole Test structure is a red herring.

/* @flow */

type Observable<T> = { observe(observer: Observer<T>): void }
type Observer<T> = { next(x: T): void }
type Subject<T> = Observable<T> & Observer<T>

var observers = []
var test: Subject<string> = {
  observe(observer) {
    observers.push(observer)
  },
  next(x) {
    for (var i = 0; i < observers.length; i++) {
      observers[i].next(x)
    }
  }
}

(EDIT: even simpler without the createSubject function.)

[samwgoldman]

Is this the same issue as identified in this intersection test?

[avikchaudhuri]

@samwgoldman This issue has been fixed (both your examples work without errors), hence closing.

There are other problems remaining with intersection types, but they could be tracked separately. (The test says what you tried should more or less work, but Subject may not be passable to an object type that has both next and observe.)