Typescript: never is accepted as a generic argument

[ac566]

TypeScript Version: 3.5.1

Search Terms:
never, generic, generics

Code

let w!: never;

function test<T>(arg: T): void { 
    console.log('hello')
}

test(w); 

My actual use-case:

type Mapper<T, U> = (value: T, index?: number, array?: T[]) => U;
type IsFunction<T> = T extends Function ? T & Function : never;

function toFunction<T, U extends keyof T, V extends IsFunction<T[U]>>(
    key: U,
): V extends Function ? Mapper<T, V extends () => infer R ? R : never> : never {
    return ((value: T) => (value[key] as V)()) as any;
}

class Test {
    x: string;
    constructor(x: string) {
        this.x = x;
    }

    getX(): string { return this.x; }
}

const y: Test[] = [new Test('hello')];
y.map(toFunction('x')) // should cause an error

Expected behavior:
I'd expect both of these situations to cause an error

Actual behavior:
No warnings or errors on compilation, even on strict mode.

[fatcerberus]

Yes, never is the bottom type. Just as unknown is a universal receiver, never is a universal producer, so you can assign it to anything.

Equivalently, never is the subtype of every other type (or if you prefer, substitutable for everything else). The empty array [] is of type never[] because you can use it to initialize any other type of array, there are no values so it doesn’t matter what other type you assign it to.

This is just the nature of a type system with both a top and bottom type. I suspect what you really want in these cases is an invalid type, in which case there’s another ticket floating around here somewhere about that...

[fatcerberus]

Found it: #23689

[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.