Type of `F<T>[keyof F<T>]` becomes unknown in TS 4.7.0-beta

[danvk]

Bug Report

I ran into this issue while testing my code with the new TS 4.7 beta. I didn't see anything in the breaking changes that looked related, so here's an issue!

🔎 Search Terms

• 4.7.0-beta

🕗 Version & Regression Information

• This changed between versions 4.6.2 and 4.7.0-beta

⏯ Playground Link

Playground Link: 4.7.0-dev showing the error and 4.6.2 not showing an error.

💻 Code

interface Bounds {
  min: number;
  max: number;
}

/**
 * A version of T where number-valued properties become Bounds-valued properties and all other
 * properties are dropped, e.g. NumericBoundsOf<{a: number, b: string}> = {a: Bounds}.
 */
type NumericBoundsOf<T> = {
  [K in keyof T as T[K] extends number | undefined ? K : never]: Bounds;
}

// Works as intended:
type X = NumericBoundsOf<{a: number; b: string}>;
//   ^? type X = { a: Bounds; }

function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
  for (const [key, val] of Object.entries(obj)) {
    const boundsForKey = bounds[key as keyof NumericBoundsOf<T>];
    if (boundsForKey) {
      const {min, max} = boundsForKey;
      if (min > val || max < val) return false;
    }
  }
  return true;
}

Output

"use strict";
function validate(obj, bounds) {
    for (const [key, val] of Object.entries(obj)) {
        const boundsForKey = bounds[key];
        if (boundsForKey) {
            const { min, max } = boundsForKey;
            if (min > val || max < val)
                return false;
        }
    }
    return true;
}

Compiler Options

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2017",
    "jsx": "react",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

🙁 Actual behavior

This code fails to type check in TypeScript 4.7.0-beta:

function validate<T extends object>(obj: T, bounds: NumericBoundsOf<T>) {
  for (const [key, val] of Object.entries(obj)) {
    const boundsForKey = bounds[key as keyof NumericBoundsOf<T>];
    if (boundsForKey) {
      const {min, max} = boundsForKey;
      //     ~~~      Property 'min' does not exist on type 'unknown'. ts(2339)
      //          ~~~ Property 'max' does not exist on type 'unknown'. ts(2339)
      if (min > val || max < val) {
        return false;
      }
    }
  }
  return true;
}

It does pass the type checker in TS 4.6.2, as I believe it should.

🙂 Expected behavior

boundsForKey should have a type of Bounds and this should pass the type checker (as it does in TS 4.6.2).

The type of boundsForKey is displayed as NumericBoundsOf<T>[keyof NumericBoundsOf<T>] in both versions, but evidently TS 4.7.0-beta resolves this to unknown whereas TS 4.6.2 resolved it to Bounds.

[RyanCavanaugh]

Bisect > #47889

[ahejlsberg]

As explained here, we can't generally simplify generic mapped types to instantiations of their template type when an as clause is present. That's what was fixed in #47889. However, when an as clause simply filters the property names (i.e. when it is a conditional type in which one branch is the iteration type variable and the other is never, or when it is an intersection that includes the iteration type variable), we could potentially still permit the simplification. Same is true if the template type doesn't reference the iteration type variable (as is the case here), though we're not equipped to consistently detect that.

I'll try to put together a PR that permits the filtering case as I think that's fairly common.