What happened

The TypeScript signature of methods like first, last, etc that take a notSetValue is:

first<NSV>(notSetValue?: NSV): V | NSV;

In TypeScript (at least in 4.1.2) if no argument is passed, NSV becomes unknown, not undefined as documented.

Possible fix:

first<NSV = undefined>(notSetValue?: NSV): V | NSV;

I found this PR but it never got merged: #1653. I think = undefined is less verbose as it doesn't create a new overload. If going with this overload approach, the correct declaration would be (instead of the one in the PR):

first(): V | undefined;
first<NSV>(notSetValue?: NSV): V | NSV;

I'd create a PR but didn't know how you wanted to handle #1653.

Note that other methods that accept a notSetValue argument probably suffer from the same issue.

Thank you.