cc @cexbrayat, this is actually breaking.

ex:

  @Directive()
  class CustomControlDir implements FormValueControl<number> {
    readonly disabled = input<boolean>(false, {transform: booleanAttribute});
  }

Hmm, this approach might not work then. Unless we widen the type for FCV toreadonly disabled?: InputSignalWithTransform<boolean, any>;?

Or maybe this should be abandoned and fixed at the TCB level, but that would require someone with better understanding of the TCB than me I'm afraid.

This is something I did look into with #66562, TransformT should be contravariant.

Isn't #67997 just undesirable in the first place; the return type InputSignalWithTransform<T, T> is really specifying that the transform function requires a T, whereas it originates from InputOptionsWithTransform<T, unknown>, so the transform's actual write type is much wider (unknown in this case).

If the return type is changed to InputSignalWithTransform<T, unknown> then it would be faithful to the transform's actual type and you wouldn't have this problem. This does, however, amplify that this specific overload is only usable for transform functions that take unknown as input type, at which point I'm not sure it's worth having it at all.