Did you try option clean-build folder ? In addition to starting with new projects, as well as on two other computers, all with the same behavior! I'm currently downloading Xcode 10 and 11 in a VM to see what's going on here. I guess bindings and xibs just aren't cool anymore.

Did you recently sandbox? My understanding is that sandboxing requires app groups. I've been using app groups since 10.10 (fuzzy, but since Extensions were introduced on macOS), because this was the only way I could get sharing user defaults to work. But it's also the time I started sandboxing everything, so that might be the cause.

Hey, I'm one of the maintainers of tidy-html5, and I'm trying to do the same thing. Did you make any progress? In my case, I'm trying to have the package reference the source, so that it will build on any platform. I'm struggling to have all of the symbols from enum.h be recognized. It looks like you're trying to build a [system] package, meaning that users will have to install the library themselves via brew or apt-get. That's common for Unix libraries, but I prefer static linking, and am struggling with Swift Package Manager. Without SPM, it's trivial to get them to build and work together in Xcode, but then that's an Xcode project and not a Swift Package.