strictFunctionTypes option should strictly check callback parameters of override methods

[falsandtru]

As @aluanhaddad mentioned at #19004 (comment).

cc @ahejlsberg

TypeScript Version: master

Code

class C {
  a(f: (a: C) => C): void {
  }
}
class D extends C {
  p: boolean = true;
  a(f: (a: D) => D): void { // should throw an error here.
    console.log(this.p);
    this.p = f(new D()).p; // type is boolean, but value is undefined.
    console.log(this.p);
  }
}

const c: C = new D();
c.a(() => new C());

Expected behavior:

error

Actual behavior:

$ node built/local/tsc.js index.ts --strictFunctionTypes && node index.js
true
undefined

[ahejlsberg]

This is working as intended. As explained here, in --strictFunctionTypes mode we still check parameters of methods and constructors bivariantly. So, there is no error here:

interface C {
  a(f: (a: C) => C): void;
}
interface D extends C {
  p: boolean;
  a(f: (a: D) => D): void;
}

But an error is reported here:

interface C {
  a: (f: (a: C) => C) => void;
}
interface D extends C {
  p: boolean;
  a: (f: (a: D) => D) => void;
}

As I explain here, we can't tighten this any further without causing Array<T> and other core types to become invariant.

[falsandtru]

I see, thanks for your explanation.