Flow: Override class instance types

[DaleLJefferson]

This is valid Typescript code, I would expect Flow to work in the same way.

Typescript
https://www.typescriptlang.org/play/index.html#src=class%20One%20%7B%20%0D%0A%20%20%20%20foo%3A%20%7Bone%3A%20string%7D%0D%0A%7D%0D%0A%0D%0Aclass%20Two%20extends%20One%20%7B%20%0D%0A%20%20%20%20foo%3A%20%7Bone%3A%20string%2C%20two%3A%20string%7D%0D%0A%7D

class One { 
    foo: {one: string}
}

class Two extends One { 
    foo: {one: string, two: string}
}

Flow

https://flow.org/try/#0PTAEAEDMBsHsHcBQiDG0CGBnToDyA7AU1AG9RFRLRJZYAuU2IhzAFwCcBLfAcwF9EA1BmygAKvFihCAD1aF8AExwFiZClRr1GzUGy68ANKFaSWHbv0FA

// @flow

class One { 
    foo: {one: string}
}

class Two extends One { 
    foo: {one: string, two: string}
}

// 8:     foo: {one: string, two: string}            
//             ^ property `two`. Property not found in
// 4:     foo: {one: string}            
//             ^ object type

[asolove]

This is expected, and indeed a major selling point for flow. Typescript does not implement variance checking for generics, but flow does.

The reason this isn’t safe is this: imagine you write a function that takes a One as an argument. It sets the foo property of that argument to an object with a one property but no two property. What should the type checker say? If you pass in a One, this is safe. But if you pass in a Two, which is a subtype of One, then this isn’t safe, because you’ll be setting the Two instance to have a foo without the property two. Therefore this code is not type safe because a runtime function call may violate the type of a Two.

Understanding all the details of variance is tricky, and probably not worth going into right here. The flow docs have some explanation of variance which should be helpful.

[DaleLJefferson]

Thanks for clearing that up @asolove

It's a feature not a bug 👍