Here the full patch attached, the previous was not applying properly.

Ccing also Stefan monnier because for some reasons the patch when
applied doesn't compile (when compiling Emacs) unless we add on
top:

    (cl--generic-prefill-dispatchers 0 (eql 'x) integer)

following the advice of the compiler, but I am not sure it is the way to
do.

Compiling register.el manually without this works correctly.