`Omit` helper loses type information when used with extended Records.

TypeScript Version: 3.8 & Nightly

Search Terms:

• Omit
• Record
• Any
• Loses type information

Expected behavior:

I expect the following types to be as described in the inline comments.

type AnyRecord = Record<string, any>;
interface ExtendsAny extends AnyRecord {
   myKey1: string;
   myKey2: string;
}
// ExtendsAny['myKey1'] === string
// ExtendsAny['otherKey'] === any

type OmitsKey = Omit<ExtendsAny, "myKey2">;
// OmitsKey['myKey'] === string
// OmitsKey['otherKey'] === any

Please note that this works without the omit, or without the record, but not when both are used together.

Actual behavior:

What actually happens is that after using omit, all keys become "any" and any information for code completion, etc that the specifically defined keys even exist, are removed, even thought they were working in the original extension.

type AnyRecord = Record<string, any>;
interface ExtendsAny extends AnyRecord {
   myKey1: string;
   myKey2: string;
}
// ExtendsAny['myKey1'] === string
// ExtendsAny['otherKey'] === any

type OmitsKey = Omit<ExtendsAny, "myKey2">;
// 🚨Problem here 🚨
// OmitsKey['myKey1'] === any
// OmitsKey['otherKey'] === any


const instance: OmitsKey = {
    myKey1: "test",
}
// 🚨 When typing this no autocomplete is provided for myKey1 🚨
instance.myKey1.

Related Issues:

• https://github.com/microsoft/TypeScript/issues/27883

Potential Workaround

In the meantime, I am redefining the base interface (very large) because it comes from a library without the record type mixed in. I do my omit, then add it back in afterwards.

Code

/**
 * @copyright 2020 Adam (charrondev) Charron
 * @license MIT
 */

export interface IBase<T = number> {
    baseProp: T;
    baseProp2: T;
    baseFunction: () => T;
}

const base: IBase<number> = {
    baseProp: 0,
    baseProp2: 0,
    baseFunction: () => 0,
};

// number
base.baseProp;

///
/// Without the omit
///
type AnyRecord = Record<string, any>;

interface IExtend<T> extends IBase<T>, AnyRecord {
    extendProp: T;
}

const extended: IExtend<number> = {
    baseProp: 0,
    baseProp2: 0,
    baseFunction: () => 0,
    extendProp: 0,
};

// number
extended.baseProp2;

///
/// With an Omit
///
interface IOmittedInterface extends Omit<IExtend<number>, "baseProp"> {}

const omitted: IOmittedInterface = {
    baseProp2: 0,
    baseFunction: () => 0,
};

// any
omitted.baseProp2;

/**
 * @copyright 2020 Adam (charrondev) Charron
 * @license MIT
 */
const base = {
    baseProp: 0,
    baseProp2: 0,
    baseFunction: () => 0,
};
// number
base.baseProp;
const extended = {
    baseProp: 0,
    baseProp2: 0,
    baseFunction: () => 0,
    extendProp: 0,
};
// number
extended.baseProp2;
const omitted = {
    baseProp2: 0,
    baseFunction: () => 0,
};
// any
omitted.baseProp2;

{
  "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