defstruct-defined classes unavailable at compile-time

[paulapatience]

Compiling the following file:

(defstruct foo)
(eval-when (:compile-toplevel :load-toplevel :execute)
  (defmethod bar ((foo foo))))

results in the following error:

ERROR while evaluating compiler-time side effect:
  Could not find the class FOO.
   [Condition of type CLEAVIR-CST-TO-AST:EVAL-ERROR]