LOOP type declaration issue

CLHS says about LOOP COUNT:

"The count construct counts the number of times that the supplied form returns true. The argument var accumulates the number of occurrences; if var is supplied, loop does not return the final count automatically. The var argument is bound as if by the construct with to a zero of the appropriate type. Subsequent values (including any necessary coercions) are computed as if by the function 1+. If into var is used, a type can be supplied for var with the type-spec argument; the consequences are unspecified if a nonnumeric type is supplied. If there is no into variable, the optional type-spec argument applies to the internal variable that is keeping the count. The default type is implementation-dependent; but it must be a supertype of type fixnum."

So FIXNUM appears to be a legal assumption, actually. Suboptimal, but legal.

By the way, in Clozure CL 1.4 it's also FIXNUM.

(BLOCK NIL
  (LET ((I MOST-NEGATIVE-FIXNUM))
    (DECLARE (TYPE NUMBER I))
    (LET ((#:LOOP-SUM-0 0))
      (DECLARE (TYPE FIXNUM #:LOOP-SUM-0))
      (ANSI-LOOP::LOOP-BODY NIL
                            (NIL NIL NIL NIL)
                            ((WHEN I (SETQ #:LOOP-SUM-0 (1+ #:LOOP-SUM-0))))
                            (NIL (ANSI-LOOP::LOOP-REALLY-DESETQ I (1+ I))
                                 (WHEN (> I '1152921504606846975)
                                   (GO ANSI-LOOP::END-LOOP))
                                 NIL)
                            ((RETURN-FROM NIL #:LOOP-SUM-0))))))

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(declaim (optimize (safety 0)))

(lambda ()
  (loop for x of-type fixnum
        from (1- most-positive-fixnum) to most-positive-fixnum
        do (progn)
        finally (print x)))

(compile nil *)

(funcall *)

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The above loops even though it is conforming ANSI CL.