Can't access indexed type using keyof when using number as an indexer

[adharris]

TypeScript Version: 2.1.6

I have a collection of entities mapped by numeric ID, and a function that plucks a single property out of the collection:

interface Foo { fooProp1: string, fooProp2: number[] }
interface Bar { barProp1: boolean, barProp: string }

interface Models {
    foos: {[id: number]: Foo}
    bars: {[id: number]: Bar}
}

function pluckProperty(models: Models, modelName: string, modelId: number, property: string) {
    ...
}

I'd like to use keyof to add strong typings to that function, but can't traverse the numeric indexer to get the indexed type. Something like this works if i index using strings instead:

function pluckProperty<K1 extends keyof Models,
                       K2 extends keyof Models[K1],
                       K3 extends keyof Models[K1][K2]>(
    models: Models, modelName: K1, modelId: K2, property: K3): Models[K1][K2][K3] {
 ...
}

This works, i think, because keyof {[key: string]: T} is string, but wont work when indexing by number because keyof {[key: number]: T} is never.

I've been able to work around this limitation by doing this:

type IdMapped<T> = {[P in keyof T]: {[id: number]: T[P]}}

interface ModelMap {
    foos: Foo;
    bars: Bar;
}   

type Models = IdMapped<ModelMap>;


function pluckProperty<K1 extends keyof Models, K2 extends keyof ModelMap[K1]>(
    models: Models, modelName: K1, modelId: number, property: K2): ModelMap[K1][K2] {
 ...
}

But that feels hackish, and if keyof is supported for string indexers then i feel it should probably work for the numeric case too.

[mhegazy]

Duplicate of #13715