Allow projectile-find-file to search in parent project

[aburlot]

My C++ program is split in two parts, ProjectA and ProjectB. ProjectA is the main framework, has all the base classes. ProjectB inherits from ProjectA and add some physics by specializing the C++ classes. Eglot can easily find references in ProjectA when I am in ProjectB thanks to compile_commands.json, but I cannot use projectile-find-file to search also in ProjectA/src. I cannot figure out a way to tell projectile that it must search somewhere else too, or that ProjectB is linked to ProjectA. I want to create a .dir-locals.el to specify it once and for all in ProjectB. Am I missing something? I can do it with eclipse, but I want to get rid of it thanks to projectile!

[aburlot]

I was not able to make use of project-vc-external-roots-function. Maybe I'm dumb, but I could not find anything relevant to make it work. I ended up coding something myself:

(defvar projectile-linked-projects nil
  "List of locations of linked projects.

Should be set in a .dir-locals.el")

(defun projectile-linked-project-files ()
  "Get a list of all files in current and linked projects."
  (cl-mapcan
   (lambda (project)
     (when (file-exists-p project)
       (mapcar (lambda (file)
                 (expand-file-name file project))
               (projectile-project-files project))))
   (list (projectile-project-root) projectile-linked-projects)))

(defun projectile-find-file-in-linked-projects ()
  "Jump to a file in the current or linked projects."
  (interactive)
  (find-file (projectile-completing-read "Find file in projects: " (projectile-linked-project-files))))

It is more or less the same than projectile-find-file-in-all-projects but with an extra variable defined in a .dir-locals.el like this

((nil . ((projectile-linked-projects . "/absolute/path/to/other/repo/"))))

Could it make its way into the main code?