Re: Info-mode patch

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> (keymap
>>  #^[nil nil keymap
>> 	#^^[3 0 nil nil nil nil nil nil nil nil nil Info-next-reference nil nil nil Info-follow-nearest-node nil nil nil nil nil nil nil nil nil nil nil nil nil
>> 	
>> I have removed rest of printed code for the brewity for the mail, but if you
>> eval above and place the cursor somewhere after the "keymap", or before the char
>> table, and then try to read the subtype with:
>> 
>> (char-table-subtype (read (current-buffer)))

Hmm... I now see that `read` does work on a sub-char-table and returns
that sub-char-table, so contrary to what I thought, they are exposed to
ELisp, and also contrary to what `lisp.h` suggests:

    /* A char-table is a kind of vectorlike, with contents like a vector,
       but with a few additional slots.  [...]
       [...] A sub-char-table appears only in an element of a
       char-table, and there's no way to access it directly from a Lisp
       program.  */

I wonder what ELisp can do with it (other than abort/crash, that is :-)

> AFAICT, this happens because the debugger now uses cl-print to display
> objects in the backtrace, and cl-print calls type-of, which aborts if
> presented with a sub-char-table:
>
>         /* "Impossible" cases.  */
> 	case PVEC_MISC_PTR:
>         case PVEC_OTHER:
>         case PVEC_SUB_CHAR_TABLE:
>         case PVEC_FREE: ;
>         }
>       emacs_abort ();

The easy fix is to make `type-of` return `sub-char-table`, of course, as
in the patch below.  But if we go that way, then we should at least
update the comment in `lisp.h`.  
Another option is to change `read` so it refuses to return
a sub-char-table.


        Stefan


diff --git a/src/data.c b/src/data.c
index 9d7e7effdcd..6de8e0cf1a1 100644
--- a/src/data.c
+++ b/src/data.c
@@ -269,10 +269,11 @@ DEFUN ("type-of", Ftype_of, Stype_of, 1, 1, 0,
 	  return Qtreesit_compiled_query;
         case PVEC_SQLITE:
           return Qsqlite;
+        case PVEC_SUB_CHAR_TABLE:
+          return Qsub_char_table;
         /* "Impossible" cases.  */
 	case PVEC_MISC_PTR:
         case PVEC_OTHER:
-        case PVEC_SUB_CHAR_TABLE:
         case PVEC_FREE: ;
         }
       emacs_abort ();
@@ -4215,6 +4216,7 @@ #define PUT_ERROR(sym, tail, msg)			\
   DEFSYM (Qvector, "vector");
   DEFSYM (Qrecord, "record");
   DEFSYM (Qchar_table, "char-table");
+  DEFSYM (Qsub_char_table, "sub-char-table");
   DEFSYM (Qbool_vector, "bool-vector");
   DEFSYM (Qhash_table, "hash-table");
   DEFSYM (Qthread, "thread");