Why does TypeScript not allow a generic enum value as default parameter value

Question

I cannot figure out why the following code confuses the compiler.

enum Enum {
    a,
    b,
    c
}

const func = <T extends Enum>(param: T = Enum.a) => param;

Here I constrain T to be of type Enum and set the default parameter to be the enumerator Enum.a. But TypeScript fails at the (param: T = Enum.a) part with the message Type 'Enum.a' is not assignable to type 'T'.

What am I missing here, for I cannot figure it out on my own and the online resources I can find only cover the basics?

Here is a playground url

Answer 1

It's an error for the same reason as the following is an error:

class Bar {
    bar = 123
}
const a = new Bar();
// Error `Bar` is not assignable to `T`
const bar = <T extends Bar>(param: T = a) => param;

Reason

Because T might be something that extends Bar and therefore Bar would not be a compatible value.

class Baz extends Bar {
    baz = 456;
}
const example:Baz = bar<Baz>(); // will blow up