* src/lread.c (read1): Allow newstyle unquote outside of backquote.

Disallow old-style backquotes inside new-style backquotes.
Don't count unquotes to figure out when we're "syntactically inside
but semantically outside of a backquote" any more.
Extend the restriction no-unescaped-commas-and-backquotes-in-symbols
to all contexts.

1 files changed, 46 insertions, 52 deletions

diff --git a/src/lread.c b/src/lread.c
index bfe3755cc51..c7b8e70963f 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -2637,7 +2637,7 @@ read1 (register Lisp_Object readcharfun, int *pch, int first_in_list)
            old-style.  For Emacs-25, we should completely remove this
            first_in_list exception (old-style can still be obtained via
            "(\`" anyway).  */
-        if (first_in_list && next_char == ' ')
+        if (!new_backquote_flag && first_in_list && next_char == ' ')
           {
             Vold_style_backquotes = Qt;
             goto default_label;
@@ -2654,33 +2654,48 @@ read1 (register Lisp_Object readcharfun, int *pch, int first_in_list)
           }
       }
     case ',':
-      if (new_backquote_flag)
-        {
-          Lisp_Object comma_type = Qnil;
-          Lisp_Object value;
-          int ch = READCHAR;
-
-          if (ch == '@')
-            comma_type = Qcomma_at;
-          else if (ch == '.')
-            comma_type = Qcomma_dot;
-          else
-            {
-              if (ch >= 0) UNREAD (ch);
-              comma_type = Qcomma;
-            }
+      {
+        int next_char = READCHAR;
+        UNREAD (next_char);
+        /* Transition from old-style to new-style:
+           It used to be impossible to have a new-style , other than within
+           a new-style `.  This is sufficient when ` and , are used in the
+           normal way, but ` and , can also appear in args to macros that
+           will not interpret them in the usual way, in which case , may be
+           used without any ` anywhere near.
+           So we now use the same heuristic as for backquote: old-style
+           unquotes are only recognized when first on a list, and when
+           followed by a space.
+           Because it's more difficult to peak 2 chars ahead, a new-style
+           ,@ can still not be used outside of a `, unless it's in the middle
+           of a list.  */
+        if (new_backquote_flag
+            || !first_in_list
+            || (next_char != ' ' && next_char != '@'))
+          {
+            Lisp_Object comma_type = Qnil;
+            Lisp_Object value;
+            int ch = READCHAR;
 
-          new_backquote_flag--;
-          value = read0 (readcharfun);
-          new_backquote_flag++;
-          return Fcons (comma_type, Fcons (value, Qnil));
-        }
-      else
-        {
-          Vold_style_backquotes = Qt;
-          goto default_label;
-        }
+            if (ch == '@')
+              comma_type = Qcomma_at;
+            else if (ch == '.')
+              comma_type = Qcomma_dot;
+            else
+              {
+                if (ch >= 0) UNREAD (ch);
+                comma_type = Qcomma;
+              }
 
+            value = read0 (readcharfun);
+            return Fcons (comma_type, Fcons (value, Qnil));
+          }
+        else
+          {
+            Vold_style_backquotes = Qt;
+            goto default_label;
+          }
+      }
     case '?':
       {
         int modifiers;
@@ -2707,26 +2722,9 @@ read1 (register Lisp_Object readcharfun, int *pch, int first_in_list)
         c |= modifiers;
 
         next_char = READCHAR;
-        if (next_char == '.')
-          {
-            /* Only a dotted-pair dot is valid after a char constant.  */
-            int next_next_char = READCHAR;
-            UNREAD (next_next_char);
-
-            ok = (next_next_char <= 040
-                  || (next_next_char < 0200
-                      && (strchr ("\"';([#?", next_next_char)
-                          || (!first_in_list && next_next_char == '`')
-                          || (new_backquote_flag && next_next_char == ','))));
-          }
-        else
-          {
-            ok = (next_char <= 040
-                  || (next_char < 0200
-                      && (strchr ("\"';()[]#?", next_char)
-                          || (!first_in_list && next_char == '`')
-                          || (new_backquote_flag && next_char == ','))));
-          }
+        ok = (next_char <= 040
+              || (next_char < 0200
+                  && (strchr ("\"';()[]#?`,.", next_char))));
         UNREAD (next_char);
         if (ok)
           return make_number (c);
@@ -2868,9 +2866,7 @@ read1 (register Lisp_Object readcharfun, int *pch, int first_in_list)
 
         if (next_char <= 040
             || (next_char < 0200
-                && (strchr ("\"';([#?", next_char)
-                    || (!first_in_list && next_char == '`')
-                    || (new_backquote_flag && next_char == ','))))
+                && (strchr ("\"';([#?`,", next_char))))
           {
             *pch = c;
             return Qnil;
@@ -2895,9 +2891,7 @@ read1 (register Lisp_Object readcharfun, int *pch, int first_in_list)
           while (c > 040
                  && c != 0x8a0 /* NBSP */
                  && (c >= 0200
-                     || (!strchr ("\"';()[]#", c)
-                         && !(!first_in_list && c == '`')
-                         && !(new_backquote_flag && c == ','))))
+                     || !(strchr ("\"';()[]#`,", c))))
             {
               if (end - p < MAX_MULTIBYTE_LENGTH)
                 {