if/typeof/in type narrowing on union type's uncommon properties

[massimonewsuk]

Code

interface Bird {
    fly: () => any;
    layEggs: () => any;
}

interface Fish {
    swim: () => any;
    layEggs: () => any;
}

function Move(pet: Fish | Bird) {
    if (pet.swim) {
        pet.swim();
    } else {
        pet.fly();
    }
    
    if (typeof pet.swim === "function") {
        pet.swim(); // Only this line works
    } else {
        pet.fly();
    }

    if ("swim" in pet) {
        pet.swim();
    } else {
        pet.fly();
    }
}

Expected behavior:
I expect the code above to work without any errors at all. I feel it's all reasonable.

Actual behavior:
I get error for all if/else branches. The only place that works is commented above.

[massimonewsuk]

Still an issue in v0.67.0

[TrySound]

post code also in flow.org/try please

[massimonewsuk]

It's easy to reproduce. Copy/paste into flow.org/try.

Here's the link for anyone that wants it:
https://flow.org/try/#0JYOwLgpgTgZghgYwgAgELCgE2QbwFDKHIwA2AngFzIAUAlMgLwB8ycIZA3AUSXGQKIBzQQGcqdRizac8AXzx5QkWIhQAxYCIAWuboREB3YAFtx9Zq3ZciyXgOFia5qVbkKYAVxAIwwAPYgyACyfgBuENQADhBgVBrayAA+aBiY9Pg2wDA00WAAdIYm6Xo2yLkFRsZ01kSyyBAkIigZpYTlpGTVJfI2JVk0YGTRftnlhcaMDAzIAESe3r4BM8WtbTEVJtXIAPTbyADyIOTIYFqatqAoBn5QANYi3fWNzSU27eRdNvJ92dQz4zNkKAyjEVqsxpVPrUnk1dKsQfkOlDCPJZEA

[massimonewsuk]

Still an issue in v0.70.0

[vkurchatkin]

Not a bug. pet.swim doesn't prove that it is a Fish, it could be a Bird with swim property of an arbitrary type.

[massimonewsuk]

@vkurchatkin Have a look at the following example which uses "exact" types instead (so Bird shouldn't have the swim property of an arbitrary type)...

type Bird = {|
    fly: () => any;
    layEggs: () => any;
|}

type Fish = {|
    swim: () => any;
    layEggs: () => any;
|}

function Move(pet: Fish | Bird) {
    if (pet.swim) {
        pet.swim(); // Works fine
    } else {
        pet.fly(); // Works fine
    }
    
    if (typeof pet.swim === "function") {
        pet.swim(); // Only this line works
    } else {
        pet.fly(); // Doesn't work - thinks pet could be a fish
    }

    if ("swim" in pet) {
        pet.swim(); // Doesn't work - thinks pet could be a bird
    } else {
        pet.fly(); // Doesn't work - thinks pet could be a fish
    }
}

[vkurchatkin]

Yes, this makes sense for exact object, but there is an issue for that already

[massimonewsuk]

@vkurchatkin Are you referring to this one? #4772

[vkurchatkin]

Yes, but I think there are others