I’ve gotten a linker error from Xcode before when it couldn’t find an extension method. I can’t remember exactly how I fixed it but it was probably some combination of {clean build, restart Xcode, delete DerivedData}.
Personally, I don’t think Swift should ever give you a linker error, unless a C/C++ header declares a function that isn’t actually implemented. So, I would consider this a bug if none of my above suggestions fix it. That being said, I also haven’t been following C++ interop too closely.
As Puyan says we have seen this one in some cases, but it is great to have a repro and know it is still there, because we could not reduce our problem, and it suddently went away with newer 5.9 improvements. In our case it was happening in our own function and the solution was providing the default parameters in every invocation place. Try to see if it goes away doing try? url.bookmarkData(options: [], includingResourceValuesForKeys: nil, relativeTo: nil).
