Yes. Changing the implementation or the doc is still in question so in the title I just use conflicts. Scanning unicodeobject.c there seems no general rules about which to use. But actually I'm in favour of ValueError. From the description in https://docs.python.org/3/library/exceptions.html, SystemError is raised when the interpreter finds an internal error. I actually don't think this error is an interpreter internal error.

But no matter what the resolution is, we can add a test for ucs4. :-)