SILGen/Sema: Avoid diagnosing @unknown default switch cases as unreachable #72213
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…hable.
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.
|
@swift-ci please test |
tshortli
deleted the
unknown-default-suppresses-unreachable-default-warning
branch
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Suppose you have an exhaustive switch statement which matches all the cases of a Swift enum defined in a different module named
External:If
Externalis compiled with library evolution andSomeEnumis not frozen, then the compiler will warn:You add an
@unknown defaultto the switch to resolve this warning. Now suppose in another build configuration,Externalis built without library evolution. The compiler will complain about the unreachability of the default case: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 defaultinstead ofdefaultis a very intentional action that would be the result of addressing the library evolution configuration, it seems reasonable to suppress theDefault will never be executeddiagnostic.