Add %default-prec.

ChangeLog

@@ -1,3 +1,22 @@
+2003-09-30  Frank Heckenbach  <frank@g-n-u.de>
+	and Paul Eggert  <eggert@twinsun.com>
+
+	* doc/bison.texinfo (Decl Summary, Contextual Precedence,
+	Table of Symbols): Document %default-prec.
+	* src/parse-gram.y (PERCENT_DEFAULT_PREC): New token.
+	(grammar_declaration): Set default_prec on %default-prec.
+	* src/scan-gram.l (%default-prec): New token.
+	* src/reader.h (default_prec): New flag.
+	* src/reader.c: Likewise.
+	(packgram): Handle it.
+	* tests/conflicts.at (%default-prec without %prec,
+	%default-prec with %prec, %default-prec 1): New tests.
+	
+2003-09-30  Paul Eggert  <eggert@twinsun.com>
+
+	* tests/testsuite.at: Include local.at, not input.at, fixing
+	a typo in the 2003-08-25 patch.
+
 2003-08-27  Akim Demaille  <akim@epita.fr>
 
 	* data/lalr1.cc (yyparse) [__GNUC__]: "Use" yyerrorlab to pacify

doc/bison.texinfo

@@ -3679,9 +3679,15 @@ Declare a terminal symbol (token type name) that is left-associative
 
 @deffn {Directive} %nonassoc
 Declare a terminal symbol (token type name) that is nonassociative
-(using it in a way that would be associative is a syntax error)
-@end deffn
 (@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@deffn {Directive} %default-prec
+Specify whether to assign a precedence to rules lacking an
+explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
 
 @deffn {Directive} %type
 Declare the type of semantic values for a nonterminal symbol
@@ -4851,6 +4857,26 @@ exp:    @dots{}
 @end group
 @end example
 
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%default-prec 0;} declaration makes it easier to discover
+this kind of problem systematically.  It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%default-prec 0;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence.  This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%default-prec 0;} can be reversed by giving
+@code{%default-prec 1;}, which is the default.
+
 @node Parser States
 @section Parser States
 @cindex finite-state machine
@@ -6782,6 +6808,12 @@ parsing.  @xref{Parser Function, ,The Parser Function @code{yyparse}}.
 Equip the parser for debugging.  @xref{Decl Summary}.
 @end deffn
 
+@deffn {Directive} %default-prec @var{state};
+Bison declaration to specify whether to assign a precedence to rules
+that lack an explicit @samp{%prec} modifier.  @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
+@end deffn
+
 @deffn {Directive} %defines
 Bison declaration to create a header file meant for the scanner.
 @xref{Decl Summary}.

src/parse-gram.y

@@ -115,6 +115,7 @@ int current_prec = 0;
 
 %token
   PERCENT_DEBUG           "%debug"
+  PERCENT_DEFAULT_PREC    "%default-prec"
   PERCENT_DEFINE          "%define"
   PERCENT_DEFINES         "%defines"
   PERCENT_ERROR_VERBOSE   "%error-verbose"
@@ -240,6 +241,13 @@ grammar_declaration:
 	symbol_printer_set (list->sym, $1, list->location);
       symbol_list_free ($2);
     }
+| "%default-prec" INT
+    {
+      if (0 <= $2 && $2 <= 1)
+	default_prec = $2;
+      else
+	complain_at (@1, _("invalid value for `%default-prec'"));
+    }
 ;
 
 symbol_declaration:

src/reader.c

@@ -41,6 +41,9 @@ merger_list *merge_functions;
 
 /* Has %union been seen?  */
 bool typed = false;
+
+/* Should rules have a default precedence?  */
+bool default_prec = true;
 
 /*-----------------------.
 | Set the start symbol.  |
@@ -409,7 +412,7 @@ packgram (void)
 	  ritem[itemno++] = symbol_number_as_item_number (p->sym->number);
 	  /* A rule gets by default the precedence and associativity
 	     of the last token in it.  */
-	  if (p->sym->class == token_sym)
+	  if (p->sym->class == token_sym && default_prec)
 	    rules[ruleno].prec = p->sym;
 	  if (p)
 	    p = p->next;

src/reader.h

@@ -73,5 +73,6 @@ void free_merger_functions (void);
 extern merger_list *merge_functions;
 
 extern bool typed;
+extern bool default_prec;
 
 #endif /* !READER_H_ */

src/scan-gram.l

@@ -182,6 +182,7 @@ splice	 (\\[ \f\t\v]*\n)*
 {
   "%binary"               return PERCENT_NONASSOC;
   "%debug"                return PERCENT_DEBUG;
+  "%default"[-_]"prec"    return PERCENT_DEFAULT_PREC;
   "%define"               return PERCENT_DEFINE;
   "%defines"              return PERCENT_DEFINES;
   "%destructor"		  token_type = PERCENT_DESTRUCTOR; BEGIN SC_PRE_CODE;

tests/conflicts.at

@@ -548,3 +548,77 @@ AT_CHECK([bison -o input.c input.y], 0, [],
 input.y: warning: expected 0 reduce/reduce conflicts
 ])
 AT_CLEANUP
+
+
+## ---------------------------- ##
+## %default-prec without %prec  ##
+## ---------------------------- ##
+
+AT_SETUP([%default-prec without %prec])
+
+AT_DATA([[input.y]],
+[[%left '+'
+%left '*'
+
+%%
+
+%default-prec 0;
+
+e:   e '+' e
+   | e '*' e
+   | '0'
+   ;
+]])
+
+AT_CHECK([bison -o input.c input.y], 0, [],
+[[input.y: conflicts: 4 shift/reduce
+]])
+AT_CLEANUP
+
+
+## ------------------------- ##
+## %default-prec with %prec  ##
+## ------------------------- ##
+
+AT_SETUP([%default-prec with %prec])
+
+AT_DATA([[input.y]],
+[[%left '+'
+%left '*'
+
+%%
+
+%default-prec 0;
+
+e:   e '+' e %prec '+'
+   | e '*' e %prec '*'
+   | '0'
+   ;
+]])
+
+AT_CHECK([bison -o input.c input.y])
+AT_CLEANUP
+
+
+## ---------------- ##
+## %default-prec 1  ##
+## ---------------- ##
+
+AT_SETUP([%default-prec 1])
+
+AT_DATA([[input.y]],
+[[%left '+'
+%left '*'
+
+%%
+
+%default-prec 1;
+
+e:   e '+' e
+   | e '*' e
+   | '0'
+   ;
+]])
+
+AT_CHECK([bison -o input.c input.y])
+AT_CLEANUP