Fix rx error with ? and ??

The ? and ?? rx operators are special in that they can be written as
characters (space and '?' respectively).  This confused the definition
look-up mechanism in rare cases.

* lisp/emacs-lisp/rx.el (rx--expand-def): Don't look up non-symbols.
* test/lisp/emacs-lisp/rx-tests.el (rx-charset-or): Test.

1 files changed, 1 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/rx.el b/lisp/emacs-lisp/rx.el
index d4a91710273..aa4b2addd47 100644
--- a/lisp/emacs-lisp/rx.el
+++ b/lisp/emacs-lisp/rx.el
@@ -134,7 +134,7 @@ Each entry is:
                 (if (cdr def)
                     (error "Not an `rx' symbol definition: %s" form)
                   (car def)))))
-        ((consp form)
+        ((and (consp form) (symbolp (car form)))
          (let* ((op (car form))
                 (def (rx--lookup-def op)))
            (and def