The precise mechanism for warning is as follows:

• The warning condition is signaled

  While the warning condition is being signaled, the muffle-warning restart is established for use by a handler. If invoked, this restart bypasses further action by warn, which in turn causes warn to immediately return nil.

• If no handler for the warning condition is found

  If no handlers for the warning condition are found, or if all such handlers decline, then the condition is reported to error output by warn in an implementation-dependent format.

• nil is returned

  The value returned by warn if it returns is nil.

  (defun foo (x)
    (let ((result (* x 2)))
      (if (not (typep result 'fixnum))
          (warn "You're using very big numbers."))
      result))
  FOO
 
  (foo 3)
 6
 
  (foo most-positive-fixnum)
 Warning: You're using very big numbers.
 4294967294
 
  (setq *break-on-signals* t)
  T
 
  (foo most-positive-fixnum)
 Break: Caveat emptor.
 To continue, type :CONTINUE followed by an option number.
 1: Return from Break.
 2: Abort to Lisp Toplevel.
 Debug> :continue 1
 Warning: You're using very big numbers.
 4294967294

*break-on-signals*, *error-output*.

If datum is a condition type, the result of (apply #'make-condition datum arguments) must be of type warning or an error of type type-error is signaled.