Function lacks ending return statement and return type does not include 'undefined' about typescript

[RyanCavanaugh]

Not really sure how to reason about this one.

On the one hand, TS assumes that if you write code, you did so for a reason. So if there's a default case, then it must be reachable via some means that you know about but we don't, so it's fair game for computing the return type. You wouldn't be allowed to return "foo"; here, for example, and the implicit return undefined; that exists at the implied exit point of the switch exists by the same logic.

On the other hand, in the absence of the default, we know the switch is exhaustive, so any code in the default block shouldn't need a return.

But again, you wrote the default block for a reason, and didn't throw out of it, so if that code isn't 100% dead, it ought to be an error. Otherwise you'll get an unsound undefined later in your program in a way that's really hard to diagnose.

I would just delete, or return or throw here for clarity. The current behavior doesn't seem wrong enough justify endangering people who do have an actually-reachable default (as is somewhat common in the case of e.g. side effects or array access)

[Mng12345]

The default case here is designed to allow the TypeScript compiler (tsc) to automatically detect when a new variant of the Size type is introduced. If the size in the default branch is of the new variant instead of never, it will trigger an error, reminding developers that a case has been missed.

[RyanCavanaugh]

You don't need one, though? https://www.typescriptlang.org/play?target=1#code/C4TwDgpgBAyglgL2gXigcgDYEMBOBzCNKAH3QDsB7HAWywyNLQGdaN6T1q4y40AoAMYUyTYFCaIUUAN4BfKFiaxJg4aKgAjDBQEBrKKgA8AJQA0APgAUAMwBcUSwEoD5qMefJX0vlF9QcEMAArjhkUNZOfLJ8qiJiAkxKqFo6upZO9mRB1BoQOC4yPn5MAO5wwAIAFg4SSM7efo1QAorQaJQ0dGj2DU19AcGhUACMRX3Rfb4tTG3Y+IQ9Y5O+AyFhAExLjROT021cPN2Fy42rQwC0w1Bbfju+0bKOQA

[Mng12345]

Yes, I can use the return type of the function as a workaround to perform exhaustive checks on the switch statement.

[Mng12345]

For absolute correctness, currently, the only way is to wrap the switch statement within a block function to ensure that no case is missed. Although typescript-eslint can also perform such checks, it's not always easy to configure typescript-eslint properly in every project.

[Mng12345]

It would be great if tsc can add an options config for switch exhausiveness check.