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, 22 insertions, 23 deletions

diff --git a/src/data.c b/src/data.c
index 3490d4985c9..752856abf09 100644
--- a/src/data.c
+++ b/src/data.c
@@ -1642,7 +1642,7 @@ void
 set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
               enum Set_Internal_Bind bindflag)
 {
-  bool voide = BASE_EQ (newval, Qunbound);
+  bool unbinding_p = BASE_EQ (newval, Qunbound);
 
   /* If restoring in a dead buffer, do nothing.  */
 
@@ -1661,10 +1661,13 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
     case SYMBOL_TRAPPED_WRITE:
       /* Setting due to thread-switching doesn't count.  */
       if (bindflag != SET_INTERNAL_THREAD_SWITCH)
-        notify_variable_watchers (symbol, voide? Qnil : newval,
-                                  (bindflag == SET_INTERNAL_BIND? Qlet :
-                                   bindflag == SET_INTERNAL_UNBIND? Qunlet :
-                                   voide? Qmakunbound : Qset),
+        notify_variable_watchers (symbol, (unbinding_p ? Qnil : newval),
+                                  (bindflag == SET_INTERNAL_BIND
+                                   ? Qlet
+                                   : (bindflag == SET_INTERNAL_UNBIND
+                                      ? Qunlet
+                                      : (unbinding_p
+                                         ? Qmakunbound : Qset))),
                                   where);
       break;
 
@@ -1682,6 +1685,11 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
     case SYMBOL_LOCALIZED:
       {
         struct Lisp_Buffer_Local_Value *blv = SYMBOL_BLV (sym);
+
+        if (unbinding_p && blv->fwd.fwdptr)
+          /* Forbid unbinding built-in variables.  */
+          error ("Built-in variables may not be unbound");
+
         if (NILP (where))
           XSETBUFFER (where, current_buffer);
 
@@ -1746,16 +1754,9 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
         set_blv_value (blv, newval);
 
         if (blv->fwd.fwdptr)
-          {
-            if (voide)
-              /* If storing void (making the symbol void), forward only through
-                 buffer-local indicator, not through Lisp_Objfwd, etc.  */
-              blv->fwd.fwdptr = NULL;
-            else
-              store_symval_forwarding (blv->fwd, newval,
-                                       BUFFERP (where)
-                                       ? XBUFFER (where) : current_buffer);
-          }
+          store_symval_forwarding (blv->fwd, newval, (BUFFERP (where)
+                                                      ? XBUFFER (where)
+                                                      : current_buffer));
         break;
       }
     case SYMBOL_FORWARDED:
@@ -1763,6 +1764,11 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
         struct buffer *buf
           = BUFFERP (where) ? XBUFFER (where) : current_buffer;
         lispfwd innercontents = SYMBOL_FWD (sym);
+
+        if (unbinding_p)
+          /* Forbid unbinding built-in variables.  */
+          error ("Built-in variables may not be unbound");
+
         if (BUFFER_OBJFWDP (innercontents))
           {
             int offset = XBUFFER_OBJFWD (innercontents)->offset;
@@ -1778,14 +1784,7 @@ set_internal (Lisp_Object symbol, Lisp_Object newval, Lisp_Object where,
               }
           }
 
-        if (voide)
-          { /* If storing void (making the symbol void), forward only through
-               buffer-local indicator, not through Lisp_Objfwd, etc.  */
-            sym->u.s.redirect = SYMBOL_PLAINVAL;
-            SET_SYMBOL_VAL (sym, newval);
-          }
-        else
-          store_symval_forwarding (/* sym, */ innercontents, newval, buf);
+        store_symval_forwarding (/* sym, */ innercontents, newval, buf);
         break;
       }
     default: emacs_abort ();