Typescript: An index signature parameter type must be 'string' or 'number' - why not its's equivalent?

[m1gu3l]

TypeScript Version: 2.2.1

Code

type LimitedFieldName = "name" | "email" | "phone";

interface ObjectWithLimitedField {
    [key: LimitedFieldName]: string;
}

// not ok

type AnyFieldName = string;

interface ObjectWithAnyField {
    [key: AnyFieldName]: string;
}

// not ok

interface ObjectWithStringField {
    [key: string]: string;
}

// ok

Expected behavior:
Index signature should allow types that evaluate to string or subset of string. The same for number.

Actual behavior:
Any literal other than string and number is explicitly disallowed.

[LOZORD]

For your first example, see Mapped Types.

type Keys = 'option1' | 'option2';
type Flags = { [K in Keys]: boolean };

WRT using string type aliases as the index type for interfaces, etc., I believe this is being worked on, but I don't remember which issue is tracking it.

[RyanCavanaugh]

See #5683

[RyanCavanaugh]

Or #7374

[m1gu3l]

@LOZORD , thanks, I must have missed that.
@RyanCavanaugh, sorry, closing.