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The compiler handles errors that happen during macroexpansion, turning
them into compiler errors. If you want to debug the error (to debug a
macro), you can set *break-on-signals* to error. For
example, this definition:
(defun foo (e l)
(do ((current l (cdr current))
((atom current) nil))
(when (eq (car current) e) (return current))))
gives this error:
; in: DEFUN FOO
; (DO ((CURRENT L (CDR CURRENT))
; ((ATOM CURRENT) NIL))
; (WHEN (EQ (CAR CURRENT) E) (RETURN CURRENT)))
;
; caught ERROR:
; (in macroexpansion of (DO # #))
; (hint: For more precise location, try *BREAK-ON-SIGNALS*.)
; DO step variable is not a symbol: (ATOM CURRENT)