Enable declaration of default %printer/%destructor. Make the parser

use these for all user-declared grammar symbols for which the user does
not declare a specific %printer/%destructor.  Thus, the parser uses it
for token 0 if the user declares it but not if Bison generates it as
$end.  Discussed starting at
<http://lists.gnu.org/archive/html/bison-patches/2006-02/msg00064.html>,
<http://lists.gnu.org/archive/html/bison-patches/2006-06/msg00091.html>,
and
<http://lists.gnu.org/archive/html/bison-patches/2006-07/msg00019.html>.
* NEWS (2.3+): Mention.
* doc/bison.texinfo (Actions in Mid-Rule): It's no longer impossible to
declare a %destructor for a mid-rule's semantic value.  It's just
impossible to declare one specific to it.
(Freeing Discarded Symbols): Mention that @$ can be used in %destructor
code.  Describe default %destructor form.
* src/parse-gram.y (grammar_declaration): Parse default
%printer/%destructor declarations.
* src/output.c (symbol_destructors_output): Use symbol_destructor_get
and symbol_destructor_location_get rather than accessing the destructor
and destructor_location members of struct symbol.
(symbol_printers_output): Likewise but for %printer's.
* src/reader.c (symbol_should_be_used): Likewise but for %destructor's
again.
* src/symtab.c (default_destructor, default_destructor_location,
default_printer, default_printer_location): New static global
variables to record the default %destructor and %printer.
(symbol_destructor_get, symbol_destructor_location_get,
symbol_printer_get, symbol_printer_location_get): New functions to
compute the appropriate %destructor and %printer for a symbol.
(default_destructor_set, default_printer_set): New functions to set the
default %destructor and %printer.
* src/symtab.h: Prototype all those new functions.
* tests/actions.at (Default %printer and %destructor): New test to
check that the right %printer and %destructor are called, that they're
not called for $end, and that $$ and @$ work correctly.
(Default %printer and %destructor for user-declared end token): New
test to check that the default %printer and %destructor are called for
a user-declared end token.
* tests/input.at (Default %printer and %destructor redeclared, Unused
values with default %destructor): New tests to check related grammar
warnings and errors.

tests/input.at

@@ -168,6 +168,65 @@ AT_CHECK_UNUSED_VALUES([1])
 AT_CLEANUP
 
 
+## --------------------------------------------- ##
+## Default %printer and %destructor redeclared.  ##
+## --------------------------------------------- ##
+
+AT_SETUP([Default %printer and %destructor redeclared])
+
+AT_DATA([[input.y]],
+[[%destructor { destroy ($$); }
+%printer { destroy ($$); }
+
+%destructor { destroy ($$); }
+%printer { destroy ($$); }
+
+%%
+
+start: ;
+
+%destructor { destroy ($$); };
+%printer { destroy ($$); };
+]])
+
+AT_CHECK([bison input.y], [1], [],
+[[input.y:4.13-29: redeclaration for default %destructor
+input.y:1.13-29: previous declaration
+input.y:5.10-26: redeclaration for default %printer
+input.y:2.10-26: previous declaration
+input.y:11.13-29: redeclaration for default %destructor
+input.y:4.13-29: previous declaration
+input.y:12.10-26: redeclaration for default %printer
+input.y:5.10-26: previous declaration
+]])
+
+AT_CLEANUP
+
+
+## ---------------------------------------- ##
+## Unused values with default %destructor.  ##
+## ---------------------------------------- ##
+
+AT_SETUP([Unused values with default %destructor])
+
+AT_DATA([[input.y]],
+[[%destructor { destroy ($$); }
+
+%%
+
+start: end end { $1; } ;
+end: { }  ;
+]])
+
+AT_CHECK([bison input.y], [0], [],
+[[input.y:5.8-22: warning: unset value: $$
+input.y:5.8-22: warning: unused value: $2
+input.y:6.6-8: warning: unset value: $$
+]])
+
+AT_CLEANUP
+
+
 ## ---------------------- ##
 ## Incompatible Aliases.  ##
 ## ---------------------- ##