Prohibit unbinding of built-in variables

* src/data.c (set_internal): Signal error if a BLV with a
redirect or a forwarded symbol is being unbound.

* test/src/data-tests.el (binding-test-makunbound-built-in): New
test.

1 files changed, 10 insertions, 1 deletions

diff --git a/test/src/data-tests.el b/test/src/data-tests.el
index a1959f62fd3..a631aabb605 100644
--- a/test/src/data-tests.el
+++ b/test/src/data-tests.el
@@ -219,6 +219,16 @@ comparing the subr with a much slower Lisp implementation."
          do (error "FAILED testcase %S %3S %3S %3S"
                    pos lf cnt rcnt)))))
 
+(ert-deftest binding-test-makunbound-built-in ()
+  "Verify that attempts to `makunbound' built-in symbols are rejected."
+  (should-error (makunbound 'initial-window-system))
+  (let ((initial-window-system 'x))
+    (should-error (makunbound 'initial-window-system)))
+  (should-error
+   (makunbound (make-local-variable 'initial-window-system)))
+  (let ((initial-window-system 'x))
+    (should-error (makunbound 'initial-window-system))))
+
 (defconst bool-vector-test-vectors
 '(""
   "0"
@@ -874,5 +884,4 @@ comparing the subr with a much slower Lisp implementation."
              ((eq subtype 'function) (cl-functionp val))
              (t (should-not (cl-typep val subtype))))))))))
 
-
 ;;; data-tests.el ends here