Actually, I notice now that even without a DEFAULT-BODY, ARGS is type-checked:

ELISP> (cl-defgeneric foo 0)
*** Eval error ***  Wrong type argument: sequencep, 0
ELISP> (cl-defgeneric foo "")
*** Eval error ***  Unrecognized usage format
ELISP> (cl-defgeneric foo [])
*** Eval error ***  Wrong type argument: listp, []

But:

ELISP> (cl-defgeneric foo nil)
nil

ELISP> (cl-defgeneric bar 'bar)
nil

(I dunno why that last one passes.)