the function '..' is not known to be defined

I was working on ox-gemini, a gemini exporter for org-mode documents. In the latest versions of emacs, string-replace is a function which replaces a word in a string. It doesn't exist in emacs 27.2 and earlier, so I added a simple shim.

;; backport for older emacs verisons
(if (not (fboundp 'string-replace))
    (defun string-replace (from to in)
      (replace-regexp-in-string (regexp-quote from) to in nil 'literal)))

The shim says "If you don't see a string-replace function, use this one). Unfortunately, this left me with this compilation error:

In toplevel form:
ox-gemini.el:286:1:Error: the function ‘string-replace’ is not known to be defined.
Makefile:5: recipe for target 'compile' failed

The solution to this is to use declare-function to say where emacs should be able to find the function. This is effectively a way to say "Emacs.. trust me.. it's around here somewhere" and squelches the error.

Here's the final code snippet.

;; backport for older emacs verisons
(if (not (fboundp 'string-replace))
    (progn
      (defun string-replace (from to in)
        (replace-regexp-in-string (regexp-quote from) to in nil 'literal))
      (declare-function string-replace "ox-gemini")
    ))