Flow: Using spread operator to create a new type with possibly undefined property creates an object where either of the property values are possible

[gajus]

type UnnormalizedEventType = {|
  name?: 'foo'
|};

type EventType = {|
  ...UnnormalizedEventType,
  name?: 'bar'
|};

const event: EventType = ({}: any);

const test = (name?: 'bar') => {};

test(event.name);

https://flow.org/try/#0C4TwDgpgBAqgdnA9gJwLYEMA2BLAXhAEwFEA3COYAFXGgF4oBvAHwCgoo51UIB+ALigByAGaJEglkwC+AbhYtQkKKXJUaUeszZQAdHvhI0WPIRUVqkADTbO3fkIBG6ZBOlyWAY0RwAzsCgQZBQCZmpK9AAUDFIC6HAgAJTuXr7+wBB+GlARtrwCgk4uCRoAfIyy8ul+EYGqOrlJQA

Expectation: No error.

Actual:

14: test(event.name);
         ^ Cannot call `test` with `event.name` bound to `name` because string literal `foo` [1] is incompatible with string literal `bar` [2].
References:
2:   name?: 'foo'            ^ [1]
12: const test = (name?: 'bar') => {};
                         ^ [2]

My understanding is that EventType should become:

type EventType = {|
  name?: 'bar'
|};

Though instead it appears to be treated as:

type EventType = {|
  name?: 'foo' | 'bar'
|};

Is this a bug/ feature?

If this is a feature, then how do I spread-copy an object without creating union of both object property types and just keeping the latest.

[gajus]

It does work as expected when the properties are not optional, e.g.

https://flow.org/try/#0C4TwDgpgBAqgdnA9gJwLYEMA2BLAXhAEwFEA3COYAFXGgF4oBvAHwCgoo51UIAuKAcgBmiRPxZMAvgG4WLUJCilyVGlHrM2UAHQ74SNFjyElFapAA0mztz78ARumRjJMlgGNEcAM7AoEMhR8JioK9AAUDBJ86HAgAJSuHt6+wBA+alBh1rwCDk5xagB8jNKyqT5h-spa2QlAA

type UnnormalizedEventType = {|
  name: 'foo'
|};

type EventType = {|
  ...UnnormalizedEventType,
  name: 'bar'
|};

const event: EventType = ({}: any);

const test = (name: 'bar') => {};

test(event.name);

How to make it work with optional properties?

[EdwardDrapkin]

You can workaround this with type WorkingSpread<A,B> = $Rest<A, B> & B; but I would guess that {...A, ...B} should resolve to the same object because they would with the ES spread operator, but they don't. Which seems like a bug.

[gajus]

ping @samwgoldman @panagosg7 @nmote

[gajus]

ping @gabelevi can this be marked as a bug?

[gabelevi]

@gajus - I think the idea here is for object type spread to mimic object spread. So you in theory should be able to type something like

function foo(objA: typeA, objB: typeB): { ...typeA, ...typeB} {
  return { ...objA, ...objB }
}

My guess is that the object spread in your example kind of works like

type UnnormalizedEventType = {|
  name?: 'foo'
|};

type EventType = {|
  ...UnnormalizedEventType,
  ...{|name?: 'bar'|},
|};

And since name is optional, Flow doesn't know whether it is present.

But yeah, as you wrote it we probably could just override the previous value. Let me ask @samwgoldman if I'm missing some edge case.

[gabelevi]

Yep! That's exactly what's happening...to make the implementation simpler we were turning the inline properties into another spread. But we totally could make the inline properties always override. I'll mark this as a bug!

[gajus]

@gabelevi Keep hitting this issue over and over. :-(

https://flow.org/try/#0KYDwDg9gTgLgBDAnmYcDKwCGMCWA7AcwEEosAVZVAXjgG8AfAKDjgGoAzAVxwBMAuOAGcYUfAQA0zNnkwBbYAOGjCklq0FZchAJKCiAG30QAxtmD84AIwgR9WPI3oBfANyNGoSLASU4AVTw8aFlMfRwAL3MMbDEScl8aBikObgslMTh6ODxOWUtgKFVpOQUhETEi9U0xXQMjUxhzAWtbe0dXd2MIPGFs4NCI4GitYlJMOBoACk5A-rDInmHYsYEAoKgQ+ajqwjjMChQASgEl3bGD6gA+OilSGE4oPBuWFgA6d5n1zcHFndGsIosLi8AQAcnYNlBUg6riAA

@samwgoldman any news on this?

The only quasi-useful solution I have found is to create a custom utility as such:

https://flow.org/try/#0KYDwDg9gTgLgBDAnmYcDKwCGMCWA7AcwEEosAVZVAXjgG8AfAKDjgGoAzAVxwBMAuOAGcYUfAQA0zNnkwBbYAOGjCklq0FZchAJKCiAG30QAxtmD84AIwgR9WPI3oBfANyNGoSLASU4AVTw8aFlMfRwAL3MMbDEScl8aBikObgslMTh6ODxOWUtgKFVpOQUhETEi9U0xXQMjUxhzAWtbe0dXd2MIPGE4CFkcGAAxVLgaAApBTksAK2BjGABKMYA+OikunvhaKRYAej24YEEwvBgAWh4cQUxLO3O8UAvT1CDzzhyNHnOAN0woQS7OBcXgCRqyMBFFgAOlhpCm+hgUicYyE0zmCzcUlIME4UDwcHhnERbg6jE2vSCUBCYUi0S0xFImFR4w+VJpESi1UIcUwAgC7NCnJ49NiTIoKEWAlFPPFCTWOxYOLxBMVLDgsOh-UGI14rMCwSFkRF3MZWEWUOBqQEAHJ2DYbcjSS4gA