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Filter object properties by type #38646

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@turolla

Description

@turolla

TypeScript Version: 3.9.2

Search Terms:
mapped type filter object properties never

Expected behavior:
The type
type FilteredKeys<T, U> = { [P in keyof T]: T[P] extends U ? P : never }[keyof T];
that allows to extract object properties by type should work everywhere.

Actual behavior:
type FilteredKeys<T, U> = { [P in keyof T]: T[P] extends U ? P : never }[keyof T];
only works externally of the declaring type

Related Issues:
#23199

Code

type FilteredKeys<T, U> = { [P in keyof T]: T[P] extends U ? P : never }[keyof T];

type FooItem<T> = T extends number ? { value: T } : undefined;

type FooMap<T> = {
    // [key in keyof T]: FooItem<T[key]>  // <-- This works
    [key in FilteredKeys<T, number>]: FooItem<T[key]>  // <-- This doesn't work
}

class Bar {
    a = 1;
    b = 2;
    c = '';    

    doSomething() {
        const fooMap: FooMap<this> = undefined!; // Omitted...
        const a = fooMap.a.value; // <-- No error expected
        const c = fooMap.c.value; // <-- Error expected
    }
}

const fooMap2: FooMap<Bar> = undefined!; // Omitted...
const a = fooMap2.a.value; // <-- No error expected
const c = fooMap2.c.value; // <-- Error expected
Output
"use strict";
class Bar {
    constructor() {
        this.a = 1;
        this.b = 2;
        this.c = '';
    }
    doSomething() {
        const fooMap = undefined; // Omitted...
        const a = fooMap.a.value; // <-- No error expected
        const c = fooMap.c.value; // <-- Error expected
    }
}
const fooMap2 = undefined; // Omitted...
const a = fooMap2.a.value; // <-- No error expected
const c = fooMap2.c.value; // <-- Error expected
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

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