Strange behavior

[Laurent Deniau]

I am using Bison 1.35 and 1.875 to parse the ISO C99 grammar and I found 
a strange behavior on a very simple case:

If I use the rule:

direct_abstract_declarator
  : '(' abstract_declarator ')'
  | direct_abstract_declarator_opt '[' assignment_expression_opt ']'
  | direct_abstract_declarator_opt '[' '*' ']'
  | '(' parameter_type_list_opt ')'
  | direct_abstract_declarator '(' parameter_type_list_opt ')'
  ;

I have only one shift/reduce conflict for all the grammar (the IF ELSE 
one). But if I use the rule

direct_abstract_declarator
  : '(' abstract_declarator ')'
  | direct_abstract_declarator_opt '[' assignment_expression_opt ']'
  | direct_abstract_declarator_opt '[' '*' ']'
  | direct_abstract_declarator_opt '(' parameter_type_list_opt ')'
  ;

I have three shift/reduce conflicts where in both cases the optional 
rules are all defined by:

THE_RULE_opt
        : /* empty */
        | THE_RULE
        ;

So while as far as I know

  | '(' parameter_type_list_opt ')'
  | direct_abstract_declarator '(' parameter_type_list_opt ')'

and

  | direct_abstract_declarator_opt '(' parameter_type_list_opt ')'

should be equivalent, the second rule introduces two new shift/reduce 
conflicts...
The good point is that the default behavior of Bison make the conflicts 
not important since they are correctly reduced (in the first case, only 
the $default reduce appear):

State 253 conflicts: 1 shift/reduce
State 297 conflicts: 1 shift/reduce

state 253

  168 abstract_declarator: pointer_opt . direct_abstract_declarator

    '('  shift, and go to state 313

    '('       [reduce using rule 233 (direct_abstract_declarator_opt)]
    $default  reduce using rule 233 (direct_abstract_declarator_opt)

    direct_abstract_declarator      go to state 314
    direct_abstract_declarator_opt  go to state 315

state 297

  145 declarator: pointer_opt . direct_declarator
  168 abstract_declarator: pointer_opt . direct_abstract_declarator

    IDENTIFIER  shift, and go to state 39
    '('         shift, and go to state 336

    '('       [reduce using rule 233 (direct_abstract_declarator_opt)]
    $default  reduce using rule 233 (direct_abstract_declarator_opt)

    identifier                      go to state 70
    direct_declarator               go to state 71
    direct_abstract_declarator      go to state 314
    direct_abstract_declarator_opt  go to state 315

Of course, the problem appears only in the full grammar, not in short 
minimalist example and it appears only for that rule while I am using 
the same _opt substitution mechanism everywhere (18 _opt rules). I can 
provide the file of the full grammar if it helps.

Regards,

ld.