Sequence function compilation fails with weird cons types

[James Kalenius]

(defun s (x) (declare (type (cons (eql 5)) x)) (car (sort x #'<)))

(s (list 5 4)) => 5. correct answer is 4.

I believe this is because the derive-type for SORT just returns the type of the sequence,
which in this case is too specific. SBCL then constant folds the whole function to 5 (and
warns that it is discarding the result of SORT).
You can see similar problems with FILL and other functions with similar derive-type methods.

This is on SBCL 2.2.5.