After updating to 3.x.x getting TS2536

[pikax]

TypeScript Version: typescript@3.3.0-dev.20181122

Search Terms:
TS2536 Record Map vuex

Code

interface ITest {
  expectedStr: string;
  expectedN: number;
}

interface MapAccessor<Getters> {
  <T = Getters, Map extends Record<string, keyof T> = Record<string, keyof T>>(
    map: Map
  ): { [K in keyof Map]: (() => T[Map[K]]) };
}

interface TestAccess {
  str: "expectedStr";
}

const t: MapAccessor<ITest> = {} as any;

const str = t({
  str: "expectedStr"
}).str;

Expected behavior:
No compile issue, is working using typescript@3.0.1, checked the breaking changes and found nothing in regards to this.

Intellisense still works as expected giving error if you change expectedStr to an invalid value.

Actual behavior:
Not giving error: index.ts:9:33 - error TS2536: Type 'Map[K]' cannot be used to index type 'T'.

Playground Link:
playground

[jack-williams]

This might be affected by this: #27490 (and thus working as intended).

Map[K] is no longer pulling in the constraint keyof T because both Map and K are generic.

I'd wait on a final say from someone on the team though,

[pikax]

What I find odd is the intellisense/editor picking up the correct value.

Is there a way to have the same behavior without the error?

[jack-williams]

Is your editor using an older version of tsserver?

[pikax]

I'm using VSCode and even the typescript playground it works (even with the error it resolves the correct type).

playground

EDIT:

[weswigham]

I think @jack-williams is right; but I'm unsure if we wanted this specific kind of pattern to break

[ahejlsberg]

This is working as intended. A simplified example:

function f1<T, U extends Record<string, keyof T>, K extends keyof U>(obj: T, map: U, key: K) {
    return obj[map[key]];  // Error
}

Here, U is constrained to an object type where properties with string names are constrained to keyof T. However, it is possible for an instantiation to supply a type argument for U that has properties with symbol names which aren't constrained in any way. Therefore U[K] isn't known to be constrained to keyof T.

Changing the constraint of U to an object type where all properties are constrained to keyof T does indeed work:

function f2<T, U extends Record<keyof U, keyof T>, K extends keyof U>(obj: T, map: U, key: K) {
    return obj[map[key]];  // Ok
}

[jack-williams]

Yep, so it seems like I was completely wrong on this one. The mistake I made was that the PR I linked to only concerned the case when relating to an indexed access type; this example is about relating from an indexed access type. Subtle, but wrong. Sorry!

[weswigham]

cc @DanielRosenwasser to double-check that we're documenting this break in the 3.2 (? I think that's the first) release.