src/lsp/predlib.lsp: SUBTYPEP fails when one of the arguments is an unfinalized class.

src/CHANGELOG

@@ -25,6 +25,10 @@ ECL 9.10.3:
 
  - ECL ignored the IGNORABLE declaration.
 
+ - ECL could not compile DEFMETHOD forms whose arguments referenced unfinalized
+   classes. The actual bug was related to SUBTYPEP not being able to determine
+   the relation between classes when one is unfinalized.
+
 * Sockets:
 
  - The socket option TCP_NODELAY option used the wrong interface: SOL_SOCKET

src/lsp/predlib.lsp

@@ -858,6 +858,8 @@ if not possible."
 	     (eq class (find-class name 'nil))
 	     (or (find-registered-tag name)
 		 (find-built-in-tag name))))
+      (and (not (clos::class-finalized-p class))
+           (throw '+canonical-type-failure+ nil))
       (register-type class
 		     #'(lambda (c) (or (si::instancep c) (symbolp c)))
 		     #'(lambda (c1 c2)