Filtering keys does not work with generic types

[Pauan]

TypeScript Version: 4.1.0-dev.20200920

Search Terms: filter key generic

Code

// Selects all the keys from T which have a value of V
type SelectKeys<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];

//================================================================
// Using a concrete type works
interface Foo {
    yes: string;
    no: number;
}

let foo: Foo = {
    yes: "hello",
    no: 5
};

// This function can only be called with keys which have a value of string
function lookup<Key extends SelectKeys<Foo, string>>(key: Key): string {
    return foo[key];
}

lookup("yes");
lookup("no"); // This error is correct

//================================================================
// Using a generic type T does not work
class Bar<T> {
    private bar: T;

    constructor(bar: T) {
        this.bar = bar;
    }

    // This method can only be called with keys which have a value of string
    lookup<Key extends SelectKeys<T, string>>(key: Key): string {
        // This error is NOT correct
        return this.bar[key];
    }
}

new Bar<Foo>(foo).lookup("yes");
new Bar<Foo>(foo).lookup("no"); // This error is correct

Expected behavior:

No error occurs.

Actual behavior:

The following error occurs:

Type 'T[SelectKeys<T, string>]' is not assignable to type 'string'.
 Type 'T[T[keyof T] extends string ? keyof T : never]' is not assignable to type 'string'.
   Type 'T[keyof T]' is not assignable to type 'string'.
     Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'string'.
       Type 'T[string]' is not assignable to type 'string'.

Playground Link: [Link]

Related Issues: #28111 #30728 #38646

[jack-williams]

See #39972 and the associated links.

[RyanCavanaugh]

👆

[typescript-bot]

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