Suggestion: Allow union in index signature if all members of union are string or number

[drew-gross]

This code is currently not allowed:

interface I {
    [index: 'a' | 'b']: number;
}

Probably because it's pretty much equivalent to this code:

interface I {
    a: number;
    b: number;
}

However, if the union is actually a type argument, it's not equivalent:

interface I<T> {
    [index: T]: number;
}

I think this would be useful. Potentially with some extra syntax to help clarify the api, something like:

interface I<T: union> {
    [index: T]: number;
}

For my specific use case, I am implementing a parsing library, and wish to have an API like this:

export type Parser<AstNodeName> = /* Some stuff involving AstNodeName */;

export interface Grammar<AstNodeName> {
    [index: AstNodeName]: Parser<AstNodeName>;
}

Which will be used like this:

type AstNodes = 'plus' | 'minus';
const myGrammar: Grammar<AstNodes> = {
    'plus': (n: AstNodes) => doSomeStuff(n),
    'minus': (n: AstNodes) => doDifferentStuff(n),
}

[weswigham]

This is probably a duplicate of #2049, if I understand your request correctly.

[drew-gross]

Yep, I think it is.