Nothing catches non-nullables passed where we expect nullables

[mjgpy3]

Non-nullable types are accepted and compile/lint just fine in our various maybe-js combinators:

const plusOne = (foo: number): null | undefined | number =>
  mmap(v => v + 1, foo)

Ideally something (tsc/lint) would disallow this (since foo is shown to be a number, i.e. NonNullable<>) in order to stunt the proliferation of unnecessary optional values.

You probably wouldn't write this directly in code most of the time. However, this poses an issue when doing any type of refactoring.

For example, in our codebase we had type Standard = ... & { domain: null | Domain } everywhere in our codebase. This entailed a bunch of uses of maybe-js to handle the optional standard.domain. Over time we made the field required, changing the type definition to type Standard = ... & { domain: Domain }. maybe not checking this left optionals and optional handling downstream of this change.