This is a known issue with Rust, for example. I have an enum with variants A and B. Somebody writes an exhaustive switch (match statement) that handles A and B with no default case. I add a variant C. Their code breaks because they don’t handle C. Adding an enum variant was a breaking change.
In Rust, the answer is #[non_exhaustive], which forces consumers to always add a default case. It’s not a huge deal, just a known issue with a well-understood solution.
I guess I don't quite get this. What I'm seeing here is that a non obvious breaking change is being turned into an obvious one.
If something is handling A and B, but you add C, the code probably needs to make sure it's handling C correctly.
I use Java in my dayjob and the behavior you've outlined is how I always code things, but it's manual and doesn't happen at compile time: I provide default that throws a runtime exception.
If your package updates and users of your package update and their code ceases to compile, that seems... fine? It's the system working as intended. They can just downgrade back to the previous known good version. It would be much worse if you made a breaking change to code but consumers' code that used yours continued to compile but no longer functioned as expected
IME the problem is the default behavior. Rust, Java, et al have that same default behavior of defaulting to closed enums, and you have to opt-in to open enums. Whether that's adding a type attribute in Rust or implementing special code in Java to handle the case. This is a footgun for distributed systems. If you don't get it wrong, then clients writing their own client-side software will get it wrong.
I'm not disparaging closed enums, they are very useful in certain contexts, but they make it really easy to do the wrong thing when reading data off the wire. Given Go is focused on this exact domain (distributed systems), I am glad the language doesn't have them.
In Rust, the answer is #[non_exhaustive], which forces consumers to always add a default case. It’s not a huge deal, just a known issue with a well-understood solution.