[SR-4196] Allow #if (conditional compilation) to guard switch cases

[belkadan]

Previous ID	SR-4196
Radar	rdar://problem/16980851
Original Reporter	@belkadan
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, LanguageFeatureRequest, StarterProposal
Assignee	@rintaro
Priority	Medium

md5: c3d4da1a290a1496adc214b1fce9223b

relates to:

• SR-4079 Availability of enum case does not propagate to its switch case scope.
• SR-2 Build configuration directives can not wrap switch cases

Issue Description:

Currently #if can only be used to guard complete declarations or complete statements, due to the (deliberate) modeling as part of the AST rather than as a textual preprocessing step. However, there's one more place where it would structurally make sense to allow #if: switch cases. This may be important if a new case is added in a new version of Swift. Right now you'd have to write the switch twice.

[belkadan]

This probably does have to go through swift-evolution, but I don't expect much pushback. Rather, I expect discussion to try to spiral out to all the other ways we could change #if, and I recommend keeping the proposal focused regardless.

[rintaro]

Implemented in Swift 4.1

[AnthonyLatsis]

PR: #9457