Override factory methods .of with extra mandatory parameters in subclasses.

[Jym77]

This is blocked until microsoft/TypeScript#4628 / microsoft/TypeScript#39699 has landed.

We make a heavy use of factory method public static of() in our classes. Unfortunately, TS doesn't let us change the signature in derived classes, even through our case is fairly safe. This has been a very discussed topic in the TS community, we should keep an eye on it and streamline code where possible.

Currently, instead of using:

class Foo {
  public static of(x: number): Foo { … }
}

class Bar extends Foo {
  // Rejected by TS due to incompatible signature of static function.
  public static of(x: number, y: number): Bar { … }
}

We use patterns like:

class Foo {
  public static of(x: number): Foo { … }
}

class Bar extends Foo {
  public static of(x: number, y: Option<number> = None): Bar { … }
}

In turn, this forces instances of Bar to store y as an option (or to have Bar.of possibly erroring at runtime without clean way of detecting cases where we don't provide y). Further down, users of Bar need some extra isSome checks when getting y, which clutters the full construction.

Another possible pattern, which keeps consumer simple but complexifies a bit the producer is

class Foo {
  public static of(x: number): Foo { … }
}

class Bar extends Foo {
  public static of(x: number): Foo
  public static of(x: number, y: number): Bar
  public static of(x: number, y?: number): Foo { 
    if (y !== undefined) {
      // do Bar specific stuff
    } else {
      return Foo.of(x)
    }
}

where the explicit overload fixes the situation.

A notable case for this is extended Diagnostic where all the extra data needs to be "optional" due to this. Sometimes we can provide a decent default (e.g. empty array), but in several cases, we need to wrap it in Option even though we know there will always be something.