Require default %destructor/%printer to be declared using

%symbol-default instead of an empty symbol list, and start working on
new per-type %destructor/%printer.  Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2006-09/msg00007.html>.
* NEWS (2.3+): Add %symbol-default to example.
* bison.texinfo (Freeing Discarded Symbols): Likewise.
(Bison Symbols): Add entry for %symbol-default.
* src/parse-gram.y (PERCENT_SYMBOL_DEFAULT): New token.
(generic_symlist, generic_symlist_item): New nonterminals for creating
a list in which each item is a symbol, semantic type, or
%symbol-default.
(grammar_declaration): Use generic_symlist in %destructor and %printer
declarations instead of symbols.1 or an empty list.
(symbol_declaration, precedence_declaration, symbols.1): Update actions
for changes to symbol_list.
* src/reader.c: Update for changes to symbol_list.
* src/scan-code.l: Likewise.
* src/scan-gram.l: Scan new PERCENT_SYMBOL_DEFAULT token.
* src/symlist.c, src/symlist.h: Extend such that a list node may
represent a semantic type or a %symbol-default in addition to just an
ordinary symbol.  Add switched functions for setting %destructor's and
%printer's.
* tests/actions.at, tests/input.at: Add %symbol-default to all default
%destructor/%printer declarations.

doc/bison.texinfo

@@ -3986,6 +3986,7 @@ For instance, if your locations use a file name, you may use
 @subsection Freeing Discarded Symbols
 @cindex freeing discarded symbols
 @findex %destructor
+@findex %symbol-default
 
 During error recovery (@pxref{Error Recovery}), symbols already pushed
 on the stack and tokens coming from the rest of the file are discarded
@@ -4012,8 +4013,9 @@ The additional parser parameters are also available (@pxref{Parser Function, ,
 The Parser Function @code{yyparse}}).
 @end deffn
 
-@deffn {Directive} %destructor @{ @var{code} @}
+@deffn {Directive} %destructor @{ @var{code} @} %symbol-default
 @cindex default %destructor
+@findex %symbol-default
 Invoke the braced @var{code} whenever the parser discards any user-defined
 grammar symbol for which the user has not specifically declared any
 @code{%destructor}.
@@ -4030,7 +4032,7 @@ For instance:
 %token <string> STRING2
 %type  <string> string1
 %type  <string> string2
-%destructor @{ free ($$); @}
+%destructor @{ free ($$); @} %symbol-default
 %destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
 @end smallexample
 
@@ -8505,6 +8507,11 @@ Bison declaration to specify the start symbol.  @xref{Start Decl, ,The
 Start-Symbol}.
 @end deffn
 
+@deffn {Directive} %symbol-default
+Used to declare a default @code{%destructor} or default @code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
 @deffn {Directive} %token
 Bison declaration to declare token(s) without specifying precedence.
 @xref{Token Decl, ,Token Type Names}.