Add %precedence support.

Unfortunately it is not possible to reuse the %prec directive.  This
is because to please POSIX, we do not require to end the rules with a
semicolon.  As a result,

foo: bar %prec baz

is ambiguous: either a rule which precedence is that of baz, or a rule,
and then a declaration of the precedence of the token baz.

	* doc/bison.texinfo: Document %precedence.
	(Precedence Only): New.
	* src/assoc.h, src/assoc.c (precedence_assoc): New.
	* src/conflicts.c (resolve_sr_conflict): Support it.
	* src/scan-gram.l, src/parse-gram.y (%precedence): New token.
	Parse it.
	* tests/calc.at: Use %precedence for NEG.
	* tests/conflicts.at (%precedence does not suffice)
	(%precedence suffices): New tests.

src/assoc.c

@@ -1,5 +1,5 @@
 /* Associativity information.
-   Copyright (C) 2002, 2005, 2006 Free Software Foundation, Inc.
+   Copyright (C) 2002, 2005, 2006, 2008 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -41,5 +41,8 @@ assoc_to_string (assoc a)
 
     case non_assoc:
       return "%nonassoc";
+
+    case precedence_assoc:
+      return "%precedence";
     }
 }

src/assoc.h

@@ -1,5 +1,5 @@
 /* Associativity information.
-   Copyright (C) 2002, 2006 Free Software Foundation, Inc.
+   Copyright (C) 2002, 2006, 2008 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -22,10 +22,11 @@
 /* Associativity values for tokens and rules.  */
 typedef enum
 {
-  undef_assoc,
-  right_assoc,
-  left_assoc,
-  non_assoc
+  undef_assoc,     /** Not defined. */
+  right_assoc,     /** %right */
+  left_assoc,      /** %left */
+  non_assoc,       /** %nonassoc */
+  precedence_assoc /** %precedence */
 } assoc;
 
 char const *assoc_to_string (assoc a);

src/conflicts.c

@@ -1,7 +1,7 @@
 /* Find and resolve or report lookahead conflicts for bison,
 
    Copyright (C) 1984, 1989, 1992, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
-   2007 Free Software Foundation, Inc.
+   2007, 2008 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -285,16 +285,21 @@ resolve_sr_conflict (state *s, int ruleno, symbol **errors, int *nerrs)
 	    flush_reduce (lookahead_tokens, i);
 	  }
 	else
-	  /* Matching precedence levels.
-	     For left association, keep only the reduction.
-	     For right association, keep only the shift.
-	     For nonassociation, keep neither.  */
+          /* Matching precedence levels.
+             For non-defined associativity, keep both: unexpected
+             associativity conflict.
+             For left associativity, keep only the reduction.
+             For right associativity, keep only the shift.
+             For nonassociativity, keep neither.  */
 
 	  switch (symbols[i]->assoc)
 	    {
-	    default:
+            case undef_assoc:
 	      abort ();
 
+            case precedence_assoc:
+              break;
+
 	    case right_assoc:
 	      log_resolution (redrule, i, right_resolution);
 	      flush_reduce (lookahead_tokens, i);

src/parse-gram.y

@@ -115,6 +115,7 @@ static int current_prec = 0;
 %token PERCENT_LEFT        "%left"
 %token PERCENT_RIGHT       "%right"
 %token PERCENT_NONASSOC    "%nonassoc"
+%token PERCENT_PRECEDENCE  "%precedence"
 
 %token PERCENT_PREC          "%prec"
 %token PERCENT_DPREC         "%dprec"
@@ -412,9 +413,10 @@ precedence_declaration:
 ;
 
 precedence_declarator:
-  "%left"     { $$ = left_assoc; }
-| "%right"    { $$ = right_assoc; }
-| "%nonassoc" { $$ = non_assoc; }
+  "%left"       { $$ = left_assoc; }
+| "%right"      { $$ = right_assoc; }
+| "%nonassoc"   { $$ = non_assoc; }
+| "%precedence" { $$ = precedence_assoc; }
 ;
 
 type.opt:

src/scan-gram.l

@@ -183,6 +183,7 @@ splice	 (\\[ \f\t\v]*\n)*
   "%output"                         return PERCENT_OUTPUT;
   "%parse-param"                    return PERCENT_PARSE_PARAM;
   "%prec"                           return PERCENT_PREC;
+  "%precedence"                     return PERCENT_PRECEDENCE;
   "%printer"                        return PERCENT_PRINTER;
   "%pure"[-_]"parser"               return PERCENT_PURE_PARSER;
   "%require"                        return PERCENT_REQUIRE;