Mapped type used with `keyof` not narrowing

[whitneyit]

TypeScript Version: Nightly

Search Terms:
computed type
mapped type narrow
keyof narrow type
type index narrow

Expected behavior:
No errors.

Actual behavior:
The type for value is not being narrowed on either side of the if statement.

Related Issues:

Code

interface Example {
    b: boolean;
    n: null;
}

function onlyBoolean(arg: boolean): void {
    // do nothing
}

function onlyNull(arg: null): void {
    // do nothing
}

function test<K extends keyof Example>(value: Example[K]): void {
    if (value === null) {
        // Argument of type 'Dict[K]' is not assignable to
        // parameter of type 'null'.
        onlyNull(value);
        return;
    }
        // Argument of type 'Dict[K]' is not assignable to
        // parameter of type 'boolean'.
    onlyBoolean(value);
}

Output

"use strict";
function onlyBoolean(arg) {
    // do nothing
}
function onlyNull(arg) {
    // do nothing
}
function test(value) {
    if (value === null) {
        // Argument of type 'Dict[K]' is not assignable to
        // parameter of type 'null'.
        onlyNull(value);
        return;
    }
    // Argument of type 'Dict[K]' is not assignable to
    // parameter of type 'boolean'.
    onlyBoolean(value);
}

Compiler Options

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": false,
    "alwaysStrict": true,
    "allowUnreachableCode": false,
    "allowUnusedLabels": false,
    "downlevelIteration": false,
    "noEmitHelpers": false,
    "noLib": false,
    "noStrictGenericChecks": false,
    "noUnusedLocals": false,
    "noUnusedParameters": false,
    "esModuleInterop": true,
    "preserveConstEnums": false,
    "removeComments": false,
    "skipLibCheck": false,
    "checkJs": false,
    "allowJs": false,
    "declaration": true,
    "experimentalDecorators": false,
    "emitDecoratorMetadata": false,
    "target": "ES2017",
    "module": "ESNext"
  }
}

Playground Link: Provided

[Cryrivers]

I'm not sure if it is relevant. TypeScript 3.7.4 (also Nightly Build) infers my variable as { [eventName in Keys]?: States | undefined; }[Keys] instead of further reducing it to States | undefined.

Playground Link

[RyanCavanaugh]

FYI this isn't a mapped type.

The function could be written as

function test(value: Example[keyof Example]): void {
    if (value === null) {
        onlyNull(value);
        return;
    }
    onlyBoolean(value);
}

without loss of generality; if you have a more complete example that requires K I could advise further.

The narrowing here can't apply because value isn't a union type -- it's a deferred lookup type over a generic type parameter. #33014 would likely be a prerequisite to handling this correctly.

[whitneyit]

Consider this more complex example:

const I18N_EXAMPLE_WITHOUT_DATA_KEY = `Example.WithoutData`;
const I18N_EXAMPLE_WITH_DATA_KEY = `Example.WithData`

type I18nKey = typeof I18N_EXAMPLE_WITHOUT_DATA_KEY | typeof I18N_EXAMPLE_WITH_DATA_KEY;

interface I18nDataMap {
    [I18N_EXAMPLE_WITHOUT_DATA_KEY]: null,
    [I18N_EXAMPLE_WITH_DATA_KEY]: { count: number },
}

function onlyNull(arg: null): void {
    // do nothing
}

function interpolate<K extends I18nKey>(key: K, data: I18nDataMap[K]): string {
    const value = `Use ${key} to fetch value from somewhere`;
    if (data === null) {
        // Argument of type 'I18nDataMap[K]' is not assignable to
        // parameter of type 'null'.
        onlyNull(data);
        return value;
    }
    const result = `Some interpolated value`;
    return result;
}

In the above example, K extends I18nKey is used to ensure the relationship between key and data is preserved. Unfortunately my initial example did not show that relationship.

[pleunv]

Apologies for digging this up, but I think I'm running into a similar (or the same) issue. Given this code (playground link):

function exhaustiveCheck(check: never): never {
  throw new Error('ERROR!');
}

interface Settings {
  settingA: boolean;
  settingB: string;
  settingC: number;
}

function getValue<T extends keyof Settings>(key: T): Settings[T] {
  if (key === 'settingA') {
    return true;
  } else if (key === 'settingB') {
    return 'test';
  } else if (key === 'settingC') {
    return 5;
  } else if (key === '') {
    exhaustiveCheck(key);
  }
}

I would expect both the key to be narrowed, and the return value to be mapped to the corresponding mapped type. However, neither seems to work:

Am I completely missing something here, or is this simply not possible? I have similar approaches in other places that do seem to work fine.