Allow `@unknown default` on all enum switching to enforce default case handling

Evolution Pitches
27

Just saw this thread after posting my question...

The @unknown default spelling looks non ideal, I'd rather prefer a bare syntax default behaving this exact way (as @8675309 pitching here)... but I guess that would be impossible without breaking compatibility:

switch someInt {
    case 0: break
    default: break // ok
}

// there's enum E { case a, b, c } somewhere

switch e {
    case .a: break
    default: break // Error or warning (as if @unknown default was written here)
}

switch e {
    case .a, .b, .c: break
    // error or warning because it's not frozen
}

switch e {
    case .a, .b, .c: break
    default: break // ok (as if @unknown default was written here)
}

I wonder though how well this (or the pitched approach) solves the general issue.. I could write default without mentioning it:

// there's enum E { case a, b, c } somewhere

switch e {
    case .a: break
    case _: break
}

The simple linter would probably allow it because it doesn't see default but it effectively is there.. Or something more elaborate:

// there's enum E { case a } somewhere
// there's enum F { case c, d } somewhere

switch (e, f) {
    case (.a, .c): break
    case (.a, _): break
}

Maybe this is a deeper feature, and not necessarily appropriate for linters (simple or complex) but could/should be embedded in the compiler itself?

There's another angle, maybe we'd also need to prohibit (discourage via a warning) having the else here at all (or even using == for enum types):

if v == .a { // like `case .a`
} else if v == .b { // like `case .b`
} else { // like `default`, but no warning?
}