FTYPE declaration for a function that returns multiple values leads to an early return on the caller side

[bohonghuang]

The following is the code where this issue can occur:

(declaim (ftype (function () fixnum) test-values))
(defun test-values ()
  (values 1 2))

(defun foo ()
  (1- (test-values)))

(foo) ; => 1, 2

It seems that ABCL treats the return type fixnum as (values fixnum) despite the slight differences between them, because when I changed it to (values fixnum fixnum), the code worked as expected.