Allan Shortlidge 3acc0d6655 SILGen/Sema: Avoid diagnosing @unknown default switch cases as unreachable. ...

Suppose you have an exhaustive switch statement which matches all the cases of
a Swift enum defined in a different module named `External`:

```
import External

var e: External.SomeEnum = //...

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

If `External` is compiled with library evolution and `SomeEnum` is not frozen,
then the compiler will warn:

```
warning: switch covers known cases, but 'SomeEnum' may have additional unknown values
```

You add an `@unknown default` to the switch to resolve this warning. Now
suppose in another build configuration, `External` is built _without_ library
evolution. The compiler will complain about the unreachability of the default
case:

```
warning: Default will never be executed
```

These contradictory compiler diagnostics encourage the developer to change the
code in a way that will cause a diagnostic in the other configuration.
Developers should have the tools to address all warning diagnostics in a
reasonable fashion and this is a case where the compiler makes that especially
difficult. Given that writing `@unknown default` instead of `default` is a very
intentional action that would be the result of addressing the library evolution
configuration, it seems reasonable to suppress the `Default will never be
executed` diagnostic.

2024-03-09 12:26:22 -08:00

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