TypeScript Version: 2.2.2

Code

// typescript code
export class Foo {
  constructor(public bar?: string) {
  }
}

// generated declaration
export declare class Foo {
    bar: string;
    constructor(bar?: string);
}

Expected behavior:
Foo.bar should be typed as string | undefined

Actual behavior:
Foo.bar is typed as string

Our declaration is no longer compatible with the source that generated it.