How to advice-add a function with no arguments to a function that takes arguments?

Say I have a functions as follows:

  (defun my/test-a (n)
    (interactive)
    (message n))
    
  (defun my/test-b ()
    (interactive)
    (sleep-for .5)
    (message "Message - B.")
    (sleep-for .5))

I then advice my/test-a with mytest-b like so: (advice-add 'my/test-a :after #'my/test-b).

However when I call (my/test-a "Message - A.") I get a "Wrong number of arguments" error. My understanding is that add-advice is feeding the argument into my/test-b, which is not expecting any arguments.

I could change my/test-b so it takes an argument and doesn't use it, but that feels very messy.

Related followup - how could I advise find-file with a function with no arguments (like my/test-b)? I understand find-file is an unusual case, because it doesn't need an argument if called interactively. But if I run (advice-add 'find-file :after #'my/test-b) and then (call-interactively 'find-file) I get a "Wrong Number Of Arguments" error again.

TIA.

Solution

You can't do that.

Your advice function has to accept the arguments for the original function.

C-hig (elisp)Advice Combinators says:

:after
Call FUNCTION after the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
(lambda (&rest r) (prog1 (apply OLDFUN r) (apply FUNCTION r)))

A way to take arbitrary arguments and ignore them is:

(defun foo (&rest _args) ...)

The underscore tells the byte-compiler that the arguments are unused in the function body on purpose.