Add new variable to force new-style backquote interpretation.

* src/lread.c (syms_of_lread): Add new variable
'force-new-style-backquotes'.
(read_internal_start): Use it.

* test/src/lread-tests.el (lread-tests--force-new-style-backquotes):
New test.

* etc/NEWS: Document new variable.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/src/lread-tests.el b/test/src/lread-tests.el
index e2d4eaa63c7..693c6c09bf3 100644
--- a/test/src/lread-tests.el
+++ b/test/src/lread-tests.el
@@ -181,6 +181,14 @@ literals (Bug#20852)."
                      (list (concat (format-message "Loading `%s': " file-name)
                                    "old-style backquotes detected!")))))))
 
+(ert-deftest lread-tests--force-new-style-backquotes ()
+  (let ((data (should-error (read "(` (a b))"))))
+    (should (equal (cdr data)
+                   '("Loading `nil': old-style backquotes detected!"))))
+  (should (equal (let ((force-new-style-backquotes t))
+                   (read "(` (a b))"))
+                 '(`(a b)))))
+
 (ert-deftest lread-lread--substitute-object-in-subtree ()
   (let ((x (cons 0 1)))
     (setcar x x)