Make tramp-archive autoloads robust for older Emacs versions

* lisp/net/tramp-archive.el (tramp-archive-file-name-regexp):
Use `eval-when-compile'.  Set it explicitly for older Emacs versions.

1 files changed, 8 insertions, 1 deletions

diff --git a/lisp/net/tramp-archive.el b/lisp/net/tramp-archive.el
index 9c1e78d6b0b..49b0c0bb6b4 100644
--- a/lisp/net/tramp-archive.el
+++ b/lisp/net/tramp-archive.el
@@ -215,11 +215,18 @@ It must be supported by libarchive(3).")
 ;; In older Emacs (prior 27.1), `tramp-archive-autoload-file-name-regexp'
 ;; is not autoloaded.  So we cannot expect it to be known in
 ;; tramp-loaddefs.el.  But it exists, when tramp-archive.el is loaded.
+;; We must wrap it into `eval-when-compile'.  Otherwise, there could
+;; be an "Eager macro-expansion failure" when unloading/reloading Tramp.
 ;;;###tramp-autoload
 (defconst tramp-archive-file-name-regexp
-  (ignore-errors (tramp-archive-autoload-file-name-regexp))
+  (eval-when-compile (ignore-errors (tramp-archive-autoload-file-name-regexp)))
   "Regular expression matching archive file names.")
 
+;; The value above is nil for Emacs 26.  Set it now.
+(if (<= emacs-major-version 26)
+    (setq tramp-archive-file-name-regexp
+          (ignore-errors (tramp-archive-autoload-file-name-regexp))))
+
 ;;;###tramp-autoload
 (defconst tramp-archive-method "archive"
   "Method name for archives in GVFS.")