FWIW, this is the biggest thing that prevents me transitioning my package documentation to DocC.
The ability to extend types from other modules (such as the standard library) with your own methods is a key feature of Swift. For example, swift-numerics extends the standard library’s floating-point types with elementary math functions.
That these extensions are entirely missing from documentation, and cannot even be referred to by other parts of the documentation, is such a significant regression that it outweighs all of the many benefits of moving to DocC.
I know that it’s being tracked; I’m only mentioning it to signal that for some packages, it’s a deal-breaker.
Edit: ah, I see. Linking and documentation are considered different features.