Bad special case with *derive-function-types* t

[Philipp Marek]

Here's a small testcase that is broken currently.

Basically, returning (values) should be compatible with (member t nil), as "no value" is as good as "NIL".

ogamita on #lisp acknowledged that it looks like a bug.

Here's the test:

(declaim (ftype (function () (values (member t nil) &optional)) values-is-a-boolean-return))
(test-util:with-test (:name :values-is-a-boolean-return)
(let* ((*evaluator-mode* :compile)
       (*derive-function-types* t)
       (compiler-result
         (with-output-to-string (*error-output*)
           (eval `(progn
                    (defun values-is-a-boolean-return ()
                      (if (zerop (random 2))
                          (values)))))
           )))
  (assert (string= "" compiler-result))))

[Stas Boukarev (stassats)]

(values) returns zero values, while the declaration declares a single value, so they're not compatible.