Add `rx-submatch-n' for compatibility (bug#37517)

It was an internal symbol in the old `rx' implementation, used in old
versions of the `flycheck' package.

* lisp/emacs-lisp/rx.el (rx-submatch-n): Alias of `rx-to-string'.
* test/lisp/emacs-lisp/rx-tests.el (rx-compat): Test it.

2 files changed, 6 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/rx.el b/lisp/emacs-lisp/rx.el
index a192ed1ad27..45fec796cc3 100644
--- a/lisp/emacs-lisp/rx.el
+++ b/lisp/emacs-lisp/rx.el
@@ -1254,6 +1254,9 @@ following constructs:
                         `(app (match-string ,i) ,name))
                       (reverse rx--pcase-vars))))))
 
+;; Obsolete internal symbol, used in old versions of the `flycheck' package.
+(define-obsolete-function-alias 'rx-submatch-n 'rx-to-string "27.1")
+
 (provide 'rx)
 
 ;;; rx.el ends here
diff --git a/test/lisp/emacs-lisp/rx-tests.el b/test/lisp/emacs-lisp/rx-tests.el
index 11de4771dea..76dcf419428 100644
--- a/test/lisp/emacs-lisp/rx-tests.el
+++ b/test/lisp/emacs-lisp/rx-tests.el
@@ -416,5 +416,8 @@
     (should (equal (rx-to-string '(seq (delta a b c) (* (epsilon d e))) t)
                    "\\(?:<(delta a b c)>\\)\\(?:<(epsilon d e)>\\)*"))))
 
+(ert-deftest rx-compat ()
+  "Test old symbol retained for compatibility (bug#37517)."
+  (should (equal (rx-submatch-n '(group-n 3 (+ nonl) eol)) "\\(?3:.+$\\)")))
 
 (provide 'rx-tests)