DESCRIBE called with string as second argument mutates that string

[Michał "phoe" Herda]

SBCL 2.0.10 on Linux amd64.

Found by zulu.inuoe.

DESCRIBE, when called with a string as its second argument, sometimes first mutates the string and only then signals a type error.

Example of a type error getting signaled at the beginning:

(let ((str (make-array 256 :element-type 'character :fill-pointer 0)))
  (describe 'describe str))

Examples of incorrect behavior:

----------------------

(describe 'describe (make-array 256 :element-type 'character :fill-pointer 0))

;=> The value
  "COMMON-LISP:DESCRIBE
  [symbol]
"

is not of type
  STREAM
   [Condition of type TYPE-ERROR]

--------------

(defun foo ()
  (make-array 256 :element-type 'character :fill-pointer 0))

(let ((str (foo)))
  (ignore-errors (describe 'describe str))
  (print str))

;=> "COMMON-LISP:DESCRIBE
  [symbol]
"