Type 'string | number' is not assignable to type 'never'. Type 'string' is not assignable to type 'never'.

[Azevedo-Dani]

Bug Report

🕗 Version & Regression Information

• Compilation error
• This is the behaviour in every version I tried. Since 3.5.1

⏯ Playground Link

Playground link with relevant code

💻 Code

interface IItem {
    id: number
    name: string
}

const item: IItem = {
    id: 1,
    name: 'item1'
}

const fun1 = (item: IItem, value: IItem[keyof IItem], name: keyof IItem) => {
    item[name] = value
}

const fun2 =  <T extends IItem>(item: T, value: T[keyof T], name: keyof T) => {
    item[name] = value
}

🙁 Actual behaviour

There's an error in fun1. We can't do item[name] = value otherwise we have the error Type 'string | number' is not assignable to type 'never'. Type 'string' is not assignable to type 'never'.

But the fun2 works perfectly.

🙂 Expected behaviour

We should have the same behaviour in fun1 and fun2.

[whzx5byb]

Related: #30769

[RyanCavanaugh]

This is correct behavior. fun1 would corrupt its item during a legal call:

const n: "id" | "name" = "id";
fun1(item, "item1", n);

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.