extends `keyof` does not allow extended new keys on interfaces since 4.3.0

[posva]

Bug Report

🔎 Search Terms

• extends keyof, missing key

🕗 Version & Regression Information

I saw this in 4.3.0 beta, it seems to exists in the nightly. It was working on 4.2.x

⏯ Playground Link

export interface GlobalOptions {
  // cannot be added or it wouldn't be able to be extended
  // suffix: string
}

// this is added by a user to extend a global customization
export interface GlobalOptions {
   // suffix: 'New'
}

export type WithSuffix<S extends string> = `${S}${'suffix' extends keyof GlobalOptions ? GlobalOptions['suffix'] : 'Suffix'}`

declare let a: WithSuffix<'hi'>

Output

export {};

Compiler Options

{
  "compilerOptions": {
    "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"
  }
}

Playground Link: Provided

### 🙁 Actual behavior

Error with `suffix cannot be used to index`

### 🙂 Expected behavior

No Error

[RyanCavanaugh]

The idiomatic way to write this would be

export type WithSuffix<S extends string> = `${S}${GlobalOptions extends Record<"suffix", unknown> ? GlobalOptions['suffix'] : 'Suffix'}`

which works as expected.

The "narrowing" of the RHS in that position was a bug and shouldn't have happened.

[posva]

Thanks a lot for telling me the correct way of doing it!