erronous COERCE type derivation in unsafe code

[Nikodemus Siivola]

Unless SAFETY=3, (COERCE X '(VECTOR 5)) isn't oblidged to actually return a vector of length 5 -- just a vector.

Our DEFTRANSFORM for COERCE takes advantage of this, but the DERIVE-TYPE method for COERCE does not take this into account.

Either the DEFTRANSFORM has to be more conservative, or the DERIVE-TYPE needs to be more careful in unsafe code about the type it returns.

Try

(funcall (compile nil
                  `(lambda (x)
                     (declare (optimize speed))
                     (let ((v (coerce x '(simple-vector 1024))))
                       (gc)
                       (list (aref v 400) (aref v 500) (aref v 700) (aref v 1000)))))
         (vector t))

for all manner of excitement.

[Nikodemus Siivola (nikodemus)]

In 1.0.43.67.