Work around certain packages not being installable from GNU ELPA on Emacs 27 and earlier.

Author: doublep

eldev-util.el

@@ -190,6 +190,18 @@ Since 1.2."
          (when ,fn
            (remove-hook 'kill-emacs-hook ,fn))))))
 
+;; May later make public, like `eldev-advised'.  For now only on usecase.
+(defmacro eldev--ensure-function (function if-missing &rest body)
+  (declare (indent 2) (debug (symbolp sexp body)))
+  (let ((present (make-symbol "$present")))
+    `(let ((,present (fboundp ',function)))
+       (unless ,present
+         (fset ',function ,if-missing))
+       (unwind-protect
+           ,(macroexp-progn body)
+         (unless ,present
+           (fmakunbound ',function))))))
+
 
 (defsubst eldev-listify (x)
   "Make a list out of X.

eldev.el

@@ -2801,7 +2801,11 @@ Since 0.2."
                                                                                                                                 (when (and flag (not (file-symlink-p filename)))
                                                                                                                                   (setf flag nil))
                                                                                                                                 (funcall original filename mode flag))))
-                                                                                      (package-install-from-archive dependency)))
+                                                                                      ;; GNU ELPA (e.g. in March 2025) itself provides packages that are not installable on older
+                                                                                      ;; Emacs versions because those lack `lisp-data-mode' and the mode is referred to in
+                                                                                      ;; e.g. `dash-pkg.el'.  Work around by temporarily providing a dummy function with that name.
+                                                                                      (eldev--ensure-function lisp-data-mode (lambda ())
+                                                                                        (package-install-from-archive dependency))))
                                                                                 (when (eq dependency-name 'eldev)
                                                                                   ;; Reload the current package again, so that we
                                                                                   ;; don't mix old and new function invocations.

test/internals.el

@@ -53,6 +53,12 @@
   ;; And only now gone completely.
   (should (equal (eldev--test-internals-inc 5) 6)))
 
+(ert-deftest eldev--ensure-function ()
+  (let ((have-lisp-data-mode (fboundp 'lisp-data-mode)))
+    (eldev--ensure-function lisp-data-mode (lambda ())
+      (should (fboundp 'lisp-data-mode)))
+    (should (eq (fboundp 'lisp-data-mode) have-lisp-data-mode))))
+
 
 (defmacro eldev--test-target-dependencies (&rest body)
   (declare (indent 0))