bug#42668: 26.1; Undefined functions when calling invoking project.el

[Philip K.]

> To make sure I understand the report: you are using project.el from
> GNU ELPA in conjunction with Emacs 26?

I see that seq has been added as a dependency in the git repository
(that appears to not have been released to ELPA yet).

The patch attached below should fix the assoc-delete-all issue.

-- 
        Philip K.

>From 4c073d987273807ff7bfb932d72c584afdad9315 Mon Sep 17 00:00:00 2001
From: Philip K <philipk@posteo.net>
Date: Sun, 2 Aug 2020 23:59:40 +0200
Subject: [PATCH] Replace assoc-delete-all with 26.1 compatible alternatives

---
 lisp/progmodes/project.el | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 51b9347bb9..06864ccf60 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -1166,7 +1166,9 @@ project-remember-project
   (project--ensure-read-project-list)
   (let ((dir (project-root pr)))
     (unless (equal (caar project--list) dir)
-      (setq project--list (assoc-delete-all dir project--list))
+      (dolist (elem project--list)
+        (when (equal dir (car elem))
+          (setq project--list (delq elem project--list))))
       (push (list dir) project--list)
       (project--write-project-list))))
 
@@ -1176,8 +1178,8 @@ project--remove-from-project-list
 result in `project-list-file'.  Announce the project's removal
 from the list."
   (project--ensure-read-project-list)
-  (when (assoc pr-dir project--list)
-    (setq project--list (assoc-delete-all pr-dir project--list))
+  (when-let ((ent (assoc pr-dir project--list)))
+    (delq ent project--list)
     (message "Project `%s' not found; removed from list" pr-dir)
     (project--write-project-list)))
 
-- 
2.20.1