Inner class of superinterface cannot be found

$ groovy playground.groovy
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
file:~/groovy-bug-inner-of-superinterface/com/foo/C.groovy: 11: unable to resolve class Level
 @ line 11, column 24.
       boolean isLoggable(Level level) {
                          ^

file:~/groovy-bug-inner-of-superinterface/com/foo/C.groovy: 16: unable to resolve class Level
 @ line 16, column 14.
       void log(Level level, ResourceBundle bundle, String msg, Throwable thrown) {}
                ^

file:~/groovy-bug-inner-of-superinterface/com/foo/C.groovy: 19: unable to resolve class Level
 @ line 19, column 14.
       void log(Level level, ResourceBundle bundle, String format, Object... params) {}
                ^

3 errors