Actually, I think the problem is using the spec

(&or symbolp lambda-expr)

instead of

[&or symbolp lambda-expr]

Because the list construct here always makes "a list with content".  So
this spec says "a list with one thing in it, either a symbol or a
lambda" which would match

(gv-define-simple-setter aref (aset))
-- 
Best regards,
  Matúš Goljer