Add the define-alien-type from the bugreport to some appropriate
impure test file, and add a WITH-TEST with a body along the lines of:
;; Used to signal an error.
(compile nil `(lambda ...with contents of TEST from the bugreport...)))
For extra credit test the returned function with inputs of both
correct and incorrect types.
Not very formal. I'm a huge fan of "poke the bug with a stick" method
of testing. If it doesn't attack you, it's probably dead. :) Over time
we accumulate more and bigger sticks.
(I can deal with this as well, but if you're out of practise...)