Define a generic function parameter that accepts any interface that extends another interface

[fleet geyser]

For some reason I can't get this working.
This is what I have now (it's example code):

export interface Car {
 id: number;
}

export interface Ferrari extends Car {
 name: 'Enzo'
}

export type DoSomethingFn = <T extends Car>(car: T) => unknown;

export const doingSomething: DoSomethingFn = (car: Ferrari) => { ... }

For some reason it's complaining that type Car doesn't match Ferrari due to its name property. T extends Car must not work like I expect?

[heady belfry]

Your problem stems from the parameter being in contravariant position

[lunar karma]

!:variance

[weak merlin]

T6#2591

`!t6:variance`:

Here's the example with Dog and Animal, explaining co/contra/in variance:

• Covariance: () => Dog is assignable to () => Animal, because Dog is assignable to Animal; it "preserves the direction of the assignability"
• Contravariance: (Animal) => void is assignable to (Dog) => void, because something that expects an Animal can also take a Dog; it "reverses the direction of the assignability"
• Invariance: (Animal) => Animal is not assignable to (Dog) => Dog, because not all returned Animals are Dogs, and (Dog) => Dog is not assignable to (Animal) => Animal, because something expecting a Dog cannot take any other kind of Animal

[heady belfry]

you’re declaring doingsomething be a function that can accept a car, then a assigning a function to it that only accepts ferraris

this is unsound

when parameters are typechecked the subtyping relation flips around

[lunar karma]

if you make the type itself generic rather than the function, that can work:

[weak merlin]

That_Guy977#5882

Playground Link

Preview:```ts
export interface Car {
id: number
}

export interface Ferrari extends Car {
name: "Enzo"
}

export type DoSomethingFn<T extends Car> = (
car: T
) => unknown

export const doingSomething: DoSomethingFn<Ferrari> = (
car: Ferrari
) => {}```

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