Class with private 'then' method can be awaited outside of the class

TypeScript Version: 4.2.0-dev.20210103, goes all the way back to 3.3.3333 and probably further

Search Terms: await private then

Expected behavior:

Expected error when awaiting C outside of the class body:

Type 'C' is not assignable to type 'PromiseLike'.
  Property 'then' is private in type 'C' but not in type 'PromiseLike'.

Actual behavior:

Instance of C can be awaited inside and outside of the class body without an error.

Related Issues: #42190

Code

class C {
    private then<T>(cb: (v: string) => PromiseLike<T> | T): Promise<T> {
        return null!;
    }
}

(async () => await new C())();

{
  "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,
    "moduleResolution": 2,
    "target": "ES2017",
    "jsx": "React",
    "module": "ESNext"
  }
}

Playground Link: Provided

Start with the provided TypeScript repro and Playground link, comparing how awaiting new C() behaves inside and outside the class. Investigate the related issue #42190 for context; done means the outside await reports that C's private then method is incompatible with PromiseLike.