`public` constructor sugar allows promoting field type to optional in subclass

[rkirov]

TypeScript Version:
nightly (1.9.0-dev.20160217)

Code

export class A {
  s: string;
}

export class B extends A {
  constructor(public s?: string) {
    super();
  }
}

Expected behavior:
My understanding is that constructor(public s?: string) is interpreted to mean the field is optional, i.e. desugars to:

  s?: string;
  constructor(s?: string) {
    this.s = s;
  }

Since subclasses can only override fields with subtypes and string | undefined is a supertype of string, this should be an error.

Actual behavior:
The code is accepted. Moreover the produced d.ts file is the following:

declare class A {
    s: string;
}
declare class B extends A {
    s?: string;
    constructor(s?: string);
}

which itself is rejected in further compilations with t.d.ts(4,15): error TS2415: Class 'B' incorrectly extends base class 'A'.