Error with interface but not with type for indexed property

[rdeneau]

TypeScript Version: 2.6.0

Code

interface I<K extends string, P> {
    [key in K]?: P; // Error: 1. A computed property name must directly refer to a built-in symbol.
                    //        2. A computed property name must be of type 'string', 'number', 'symbol' or 'any'.
}

type T<K extends string, P> = {
    [key in K]?: P; // No Errors !?
};

Expected behavior:
Same error for both definitions, I and T.

Actual behavior:
Error only with the interface.

[aluanhaddad]

This is essentially a duplicate of #18299

[kreba]

For other n00bs like me: Note that there is a difference between "indexed" and "mapped" types.
See http://www.typescriptlang.org/docs/handbook/advanced-types.html#mapped-types

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.