Can't index object in generic function with property name extracted with Pick from derived type

[popadi]

TypeScript Version: 4.1.0-beta

Search Terms:
pick, generic function, keyof

Code

interface IRecord {
  id: number,
  name: string
}

type FilterProperties<T> = Pick<
  T,
  { [K in keyof T]: T[K] extends string ? K : never }[keyof T]
>;

function extract<T>(data: T[], field: keyof FilterProperties<T>): void {
  const record: T = data[0];
  const value: string = record[field]; // see error below
}

const data: IRecord[] = [{ id: 1, name: "one" }, { id: 2, name: "two" }]

extract<IRecord>(data, "id"); // expected: error that id can't be used as a parameter since it's not a string
extract<IRecord>(data, "name") // no error, expected behaviour

Expected behavior:
I'm trying to create a function that receives an object/an array of objects of type T and a field name but the user should be able to call the function only if the type of that field in T is string. FilterProperties is just a type (union) that picks those properties from T and can be easily extended to filter any kind of properties.

Since I was able to call the function correctly on the last line (because name is a field of type string, I was expecting to be able to extract the value in the function too, but I get the error below. Everything works if you define everything explicitly (no generics).

Actual behavior:

Type 'T[{ [K in keyof T]: T[K] extends string ? K : never; }[keyof T]]' is not assignable to type 'string'.
  Type 'T[T[keyof T] extends string ? keyof T : never]' is not assignable to type 'string'.
    Type 'T[keyof T]' is not assignable to type 'string'.
      Type 'T[string] | T[number] | T[symbol]' is not assignable to type 'string'.
        Type 'T[string]' is not assignable to type 'string'.ts(2322)

Playground Link:
https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgJICUIIPZQCbIDeAsAFDLLB4BcyIArgLYBG0ANGRSHIxLQM5gooAOZkAvmTJgAngAcUAMWAAbSFAAKUbAqhhgEfgB4AKgD5kAXmQbgCANZHOyEx3JFkAbQDSlEMnsIGWwYFwBdWhMfMOQIAA9IEDx+ZEFhEBFkAH5kX1oQCAA3aGRxT0Dg0JMwsjMAbilSGHoQBH1sf3ihRDBTMwAKPDgwOEjPMLZkGAMVGgCgkORlNWgtHWh9Qz6ASlpC7CoiZxwQQWQoLFw5kytkIZHPAAYwhvcTs8K4FXo+VKFRW4XHD4TzTCCzF7IAD0UNSEBQ0G0UGQrBU2AA7hJGu8wHdhqM0JhgXhxrdPIRKHMAIyTbi8WgAIg6EAZpUmFKotAATLSeL8GWB0dhWeIaqQyF0oD0jBhLvgBvc4JMGVQGds6tDYYjcMgwAALYaU5AIOAgADkuNYyHo-AgBDgKTgyDkcClvHUqVASEoYDNKRA2FxTrSoglCSlbRlRKuCvxyrpLO2mro2FiUCRk3iCjadpREANhWA2HoUCAA

Let me know if I should update the title to be more descriptive.

[MartinJohns]

You're assuming that T extends string means "T is a string", but that's not the case. It just extends string, but it can be a more narrow literal string type, e.g. 'foo'. Trying to assign 'foo' to string is not allowed.

edit: It was a long day and this was completely wrong.

[RyanCavanaugh]

@MartinJohns that's a valid subtyping

The problem here is that we don't have the higher-order reasoning to say "If a key came from an operation that filters out keys that aren't strings, then indexing by that key must have produced a string". The FilterProperties type is effectively logically opaque from TS's point of view.

An alternate way of writing the function that achieves the desired semantics would be:

interface IRecord {
  id: number,
  name: string
}

function extract<K extends string>(data: Record<K, string>[], field: K): void {
  const value: string = data[0][field]; // see error below
}


const data: IRecord[] = [{ id: 1, name: "one" }, { id: 2, name: "two" }]

extract(data, "id"); // error that id can't be used as a parameter since it's not a string
extract(data, "name") // no error, expected behaviour