[swift-evolution] warnings for out of scope?

[Joe Groff]

> On Jan 25, 2017, at 10:40 AM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This is contrary to several deliberate design decisions, if I understand correctly.
> 
> First, revisions to visibility rules in the Swift 3 timeline were made with the deliberate intention that it should be possible to model greater visibility within a type (e.g. public members) without actually exporting that type. As Swift does not have optional warnings that can be turned off, it would be incongruous if the language also warned users away from creating internal types or variables before they are used. Unlike warnings about unused variables within a scope, which are by definition local, a warning such as proposed would be much more disruptive.
> 
> Second, a variable with no access modifier defaults to internal, and this is deliberate for the purpose of progressive disclosure (i.e. it is, by design, possible for a new user to write useful code separated across multiple files without learning what access modifiers are). This would be undone if nearly every such use prompted a warning.
> 
> In summary, I think the issue here is more one of style than safety, and IMO is more within the scope of a linter.

One place a warning like this would be useful is with private/fileprivate code that resulted from migrating Swift 2 to 3. Xcode's automatic migrator naively changed all Swift 2 private declarations to fileprivate, since that's the obvious semantics-preserving change, but it's possible that this has had the knock-on effect that people overuse "fileprivate" because that's the example set by the migrator, and not for technical reasons. Given the number of ideas that have been raised about further extending or tweaking the visibility model since Swift 3, it's clear there's still some dissatisfaction with our current model, and we've been trying to get clear information about how well the existing model is working. Fileprivate is potentially overrepresented in code in the wild due to the migrator's behavior and people cargo-culting the migrator's code patterns, so a warning that suggested to users when they could make use of 'private' might help steer people to clean up their migrated code and give us a better idea of how well the model fits real-world problems.

-Joe