Internal Typealias Promotion

In some scenarios it might be useful to declare a typealias internally to a module, to make it easier to implement the functionality of the module itself, but less useful to export that typealias to clients of the module. For example, consider an image manipulation framework that can work with NSImage or UIImage instances, depending on the platform. Internally to the module, I might define:

#if os(macOS)
	typealias RSPlatformNativeImage = NSImage
	typealias RSPlatformNativeImage = UIImage

Then I can implement methods, including public facing methods like:

public func monochromeImage(fromImage image: RSPlatformNativeImage) -> RSPlatformNativeImage { ... }

Since I haven’t marked my typealiases as ‘public’, they won’t be exported to clients, but the above will also fail to compile. Swift requires that public methods work only with public types. This makes sense, because if the types aren’t public, how are clients expected to be able to work with them?

But if I mark the typealiases public, I impose a new type “RSPlatformNativeImage” on clients, when as far as they are concerned, this method operates on either an NSImage or UIImage. They might quickly get the idea that RSPlatformNativeImage is just a typealias, but it’s a bit of unwanted clutter on the public-facing API.

Obviously I can solve this by adding more platform-specific directives to the module so that whole functions are declared as working with either NSImage or UIImage, but it would be nice if Swift would help me out here. Instead of giving a compiler error, Swift could simply export the method using the public type that the typealias resolves to. In which case a client of the module for iOS would see the method as:

public func monochromeImage(fromImage image: UIImage) -> UIImage { ... }

And for Mac:

public func monochromeImage(fromImage image: NSImage) -> NSImage { ... }

Handling internal typealiases like this would cause the behavior for Swift clients to match what is already being done for Objective-C clients. The generated Module-Swift.h for this method in a Mac project is:

- (NSImage * _Nonnull) monochromeImageFromImage:(NSImage * _Nonnull)image SWIFT_WARN_UNUSED_RESULT;

Thus for Objective-C clients the clutter of the typelias definition is tidied away, but for Swift clients, it must still be dealt with. I filed a bug requesting this behavior in the Swift bug tracking system.