Union type can incorrectly fail when options have same-named but differently-typed property

[Macil]

If you have a union type where the first (by alphabetically) property of the first option has the same name as a property on a later option but with a different type, and then you have an object of the second type that has a value for that property which happens to also fit the type of the first option's first property, then Flow incorrectly gives a type error instead of realizing that the value belongs to the second option.

/* @flow */

var bar: Array<{b: ?boolean, c: number} | {b: boolean}> = [
  {b: true, c: 123},
  {b: true}
];

[chris /tmp/foo]$ flow version
Flow, a static type checker for JavaScript, version 0.21.0
[chris /tmp/foo]$ flow
bar.js:5
  5:   {b: true}
       ^^^^^^^^^ object literal. This type is incompatible with
  3: var bar: Array<{b: ?boolean, c: number} | {b: boolean}> = [
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ union: object type(s)


Found 1 error

The following changes each cause Flow to correctly not give any errors:

• Re-ordering the union type.

var bar: Array<{b: boolean} | {b: ?boolean, c: number}> = [

• Changing the first option's b property to match the type of the b property on the second option.

var bar: Array<{b: boolean, c: number} | {b: boolean}> = [

• Changing the first option to have a required property alphabetically before b. ("c" -> "a" in the example.)

var bar: Array<{b: ?boolean, a: number} | {b: boolean}> = [
  {b: true, a: 123},
  {b: true}
];

I'm not familiar with Flow's own code, but I'd make a wild guess that Flow is incorrectly choosing not to backtrack after it sees a property in an object fit a property in an option and no further options have a property of the same exact type, even though further options might have a same-named property of a compatible type.

[avikchaudhuri]

Yes, this is a long-standing known problem in the implementation of union types that we're working hard to fix. Your diagnosis is correct, btw.

[grncdr]

Maybe this can be added #1331 ?

[Macil]

That's about intersection types, not union types. I don't know if this issue has much in common technically with those.

[avikchaudhuri]

This will be fixed in an upcoming release.