Optional chaining should always include undefined in its resolved type

[Svish]

TypeScript Version: 3.7.5

Search Terms: optional chaining return type

Code

I have a formatting function, where the input should always be a string, but in certain cases, from the backend, it's undefined for some reason. So to make sure the front-end at least doesn't crash when that happens, I added optional chaining, which fixed the issue.

But the types are kind of off now, and I didn't realize until now because Typescript didn't complain about anything.

export const formatAccountNumber = (account: string): string =>
  account?.replace(/^(\d{4})(\d{2})(\d{5})/, '$1 $2 $3');

Expected behavior: Should get an error/warning about something here. Preferably, I guess optional chaining should always expand the type to include undefined to whatever it is added to? Then, in this example, Typescript could say that the return type is string, but I'm trying to return string | undefined? Either that, or maybe it shouldn't be possible to use optional chaining on a value that does not include either the type undefined and/or null?

Actual behavior: No warnings or errors.

Related Issues: no

[nmain]

This is the first discussion point of #36623. Also see #36163.

[awjreynolds]

Just to say this one caught us out in our codebase and took a while to figure out as it wasn't being picked up.