Trouble creating Swift library package around system library

[SDGGiesbrecht (Jeremy David Giesbrecht)]

There is @_exported import SomeModule, which is used frequently within the Swift project, but is underscored because its spelling and some semantic details still aren’t finalized. You can use it safely as long as you don’t care about source compatibility with future releases of Swift.

@_exported and fixing import visibility

I told @Jon_Shier I'd write something up on @_exported and visibility in general, so here it is. It's a bit of a brain dump, but I know if I wait until I have time to edit it it won't get posted for months. (That's already true, in fact; I've had this model kicking around my head for probably close to a year now.) So, thanks for the prompt, Jon. Today, if your Swift library "Foo" uses a library "Bar", the headers (or swiftmodule files) of "Bar" must be available to all clients of "Foo". This i…