[SR-7303] "Method must be declared private because its parameter uses a private type", although the method is in a private extension

[swift-ci]

Previous ID	SR-7303
Radar	None
Original Reporter	theo (JIRA User)
Type	Bug

Attachment: Download

Environment

Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)
Target: x86_64-apple-macosx10.9

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AccessControl
Assignee	None
Priority	Medium

md5: aca964c8881a8f4aaecf2590b24bb72f

is duplicated by:

• SR-7420 Private enum in private extension cannot be accessed in method
• SR-7750 Private extensions can't use private types in method declarations
• SR-8500 Method defined in private extension unable to use private type from within the same access control group

Issue Description:

The following piece of code produces this error: Method must be declared private because its parameter uses a private type

class Foo {

  private enum MyPrivateEnum {
    case myCase
  }

}

private extension Foo {

  func bar(argument: MyPrivateEnum) {

  }

}

[belkadan]

This is "technically correct" but certainly confusing, and probably needs a better diagnostic. private means "private to the enclosing scope", and so if you write it on an extension you're really saying the same thing as fileprivate (because at the point where you wrote private, there's no enclosing scope).

Arguably, though, this behavior is not useful, and so while changing Swift's access control model is a topic certain to draw flurries of discussion, it's something that could be changed in a very focused way through the Swift Evolution Process. (This was discussed somewhat in the acceptance thread for SE-0169, which first split up private and fileprivate.)