[BUG] Type parameter that declared by "extends of" should not be a required type parameter.

[yaquawa]

Here is the code I've got an error.

class Foo<T, K extends keyof T> {
    sayHi(name: K | K[]) {
        console.log(`hi ${name}`);
    }
}

interface Event {
    click(element: string, event: object):void
    touch(element: string, event: object):void
}

const f = new Foo<Event>(); // TS2558: Expected 2 type arguments, but got 1.

Where K is a type that is determined by T automatically, so typescript compiler should not allow users to specify the K type parameter manually.

Did I miss any use cases that a user should specify K manually?

If the K can be an "optional type parameter" that can be specified by user manually , the format should be this I suppose.

class Foo<T, K = keyof T> {
}

By now I'm using this format to avoid the error. But I think this is so pointless..

class Foo<T, K extends keyof T = keyof T> {
}

[RyanCavanaugh]

See #16597 and others

[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.