curiosity in value of "source-directory" in function "autoload-generated-file" and in the containing file "autoload.el.gz"

[Alan]

I find it curious that the file 
"/Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/autoload.el.gz" has 
the following code:

(defun autoload-generated-file ()
  (expand-file-name generated-autoload-file
                    ;; File-local settings of generated-autoload-file should
                    ;; be interpreted relative to the file's location,
                    ;; of course.
                    (if (not (local-variable-p 'generated-autoload-file))
                        (expand-file-name "lisp" source-directory))))

and "source-directory" in this code has no other mention in this file, except 
in a comment.

I am debugging difficulties creating the file "vm-autoloads.el" using the View 
Mail machinery that allows one to update to the lastest trunk version of VM.  
In doing so I run into a situation where "source-directory" has the value

"/Users/build/workspace/Emacs-Multi-Build/label/mavericks/emacs-source/lisp/vm-autoloads.el"

& this directory is unavailable (for good reason).

My system is:

Mac Book Air running OS X 10.12.5 Sierra

GNU Emacs 25.1.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version 10.9.5 
(Build 13F1911)) of 2016-09-20

I won't try to describe what I am doing to resolve the issues getting 
"vm-autoloads.el" built properly.  I only stop to remark about this curiosity 
that I see in the value of "source=directlry" when "autoload-generated-file" 
gets used.

For what it is worth, the posting at URL 
"https://stackoverflow.com/questions/10535529/emacs-elisp-expand-file-name-behaviour-on-windows";
 shows a similar difficulty with the value of "source-directory" in this 
function.