The code snippet below crashes if you run it a few times on the iPhone XS simulator. I've also attached a sample project demonstrating the issue.
Changing the name of the var with the ``s in it can cause the crash to occur more or less often. I'm not actually sure if the `` is directly responsible, but since it seems to have a special significance in name mangling, I assume that the objc selector is name-mangled, and the code for `appending` seems to be doing some kind of manipulation of the selector name I'm guessing it's related.
The crash is usually similar to what's reproduced below: