SE-0396: Conform Never to Codable

[itaiferber]

@jeremyabannister

Am I correct in thinking that to have this behavior we would need modify the synthesized Codable conformance for enums to handle Never in a special way?

Yes, that's correct.

1. For non-generic enums, the compiler can statically know whether a case can be written off during synthesis
2. For generic enums, the types would need to be inspected at runtime, since the generics can be instantiated externally; synthesis would need to be updated in the compiler to check all generic types. This may come at an unfortunate performance cost for a very rare scenario

It does get a bit trickier, if you want to take this idea further:

Never is not the only non-instantiable type, but we don't currently to my knowledge have a formalized concept of "non-instantiable", either in the compiler or at runtime. For example:

enum MyNever {
    private init() { fatalError() }
}

struct MyNonInstantiableStruct {
    // Non-instantiable, because of either type:
    let never1: Never
    let never2: MyNever
}

The compiler knows about Never, but it doesn't know about MyNonInstantiableStruct — so .b(Never) could be avoided during decoding, but .b(MyNonInstantiableStruct) could not.

Obviously, MyNonInstantiableStruct is a useless type here, but you can easily end up with a Never deeply nested within a type hierarchy, that in the general case, the compiler would need to search for if we wanted to generalize this concept.

@jeremyabannister

A possible approach would be to consider that from an encoding/decoding perspective the type Result<Int, Never> does not have the failure case, meaning that it should behave identically to whatever we do in the below case:

I think whether we should formalize this is a good question, both philosophically and practically. It's easy to do the bare minimum here and get rid of the most obvious of foot-guns, but being 100% consistent would be tricky and take more work, potentially.

Though, handling this could easily be additive — and it's quite possible that it's simply enough to have Never throw and call it a day.