Cannot able create a Record from another Record (With all optional keys)

I can hypothesize that the problem could be related to some kind of TS type or some strict runtime check. In other words it could be something like the following:

interface B {
  t?: string;
  l?: string;
  p?: string;
}

is different from

interface B {
  t?: string | undefined;
  l?: string | undefined;
  p?: string | undefined;
}

Or, related to runtime check, t in a gives different results than a.t !== undefined for those cases.

Yes, because if null is a one million dollar mistake, JS had the nice idea to have 3 different ways of expressing a slightly different variant of a null value (undefined, null and the absence of the field).

I would like to get an answer from the OP as well, but at the same time I am curious about possible approaches in Rescript (other than going %raw). Said that, there are a lot of JS projects out there that don’t even distinguish a null from a 0 or an empty string, so…

EDIT: just to throw ideas, this should work, but obviously it requires a bit of boilerplate.