CODEWITHSUNDEEP
typescriptvariancecontravariant
Eric Chen
Eric Chen

Reputation: 95

Why does Typescript allow assignability for a generic type parameter in a contravariant position?

In the following code, the type parameter T appears in a contravariant position in the Setter type. Why does typescript allow the assignment in a way that seems to break variance rules?

type Setter1<T> = {
    set(val: T): void
}

function f1(x: Setter1<'a'>): void {
    const y: Setter1<string> = x // no compile error. why?
}

On the other hand, the below code does have a compile error (which is what I would expect).

type Setter2<T> = {
    set: (val: T) => void
}

function f2(x: Setter2<'a'>): void {
    const y: Setter2<string> = x // compile error. good!
}

Upvotes: 1

Views: 54

Answers (0)

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