Cannot be used to index type in 3.9 (worked in 3.8)

[jasonkuhrt]

TypeScript Version: 3.9.3

Search Terms:

cannot be used to index type

Code

2536

// A *self-contained* demonstration of the problem follows...

// works in 3.8, fails in 3.9
// Is this a bug in 3.9?
// Nexus (nexusjs.org) depended on this functionality

type Get1<
  A extends any,
  B extends any,
  C extends 'filtering' | 'ordering'
> = A extends keyof Stuff
  ? B extends keyof Stuff[A]
    ? Stuff[A][B][C] // error here
    : never
  : never;

interface Stuff {
  // [k:string]: any               // <-- Putting this here fixes Get1 in TS 3.9
  Blog: {
    // [k:string]: any               // <-- Putting this here fixes Get1 in TS 3.9      
    posts: {      
      filtering: 'a',
      ordering: 'b'
    }
  }
}

Expected behavior:

TS 3.9 supports the same type logic as in 3.8 (see playground repro)

Actual behavior:

Breaks in 3.9 (our context: graphql-nexus/nexus#859)

Playground Link:

link

[ahejlsberg]

The immediate cause of this is #29571. Previously, a type parameter declared with extends any was treated as just any, which effectively turned off type checking. For example, you could change Stuff[A][B][C] to Stuff[A][42]['elephant'] in your example with no error!

We now treat extends any the same as having no constraint, meaning that things actually get checked. And, in your example A might not include 'Blog' and B might not include 'posts', so there is no guarantee that 'filtering' | 'ordering' is a valid key for Stuff[A][B]. You need to include another check to make it work:

type Get1<
  A extends any,
  B extends any,
  C extends 'filtering' | 'ordering'
> = A extends keyof Stuff
  ? B extends keyof Stuff[A]
    ? C extends keyof Stuff[A][B]
      ? Stuff[A][B][C] // Ok
      : never
    : never
  : never;

[jasonkuhrt]

Ahh thanks @ahejlsberg I thought I tried that 🤦.

I also mistakenly thought that TS would statically analyze that Stuff only had filtering and ordering. I guess though the only way for that to happen would be having Stuff come through as a constrained generic.

There's probably an obvious reason TS must/does behave this way, but I don't see it.

Anyways, getting off-topic here.

Thanks again @ahejlsberg!