启用Bison的parse.lac full选项后错误恢复失效问题求助

为了避免生成不正确的错误消息，按照Bison文档说明启用了parse.lac full选项，但启用后错误恢复功能无法正常工作。

示例代码 #

%{
    #include <stdio.h>
    int yylex();
    void yyerror(const char *);
%}

%define parse.lac full
%define parse.error detailed

%%

prog:
      init statements { printf("program finished successfully!\n"); }
    ;

init:
      %empty
    | '!' '\n' { printf("an optional \"special\" first line\n"); }
    ;

statements:
      %empty
    | statements 'x' '\n' { printf("correct line\n"); }
    | statements error '\n' { yyerrok; printf("error line\n"); }
    ;

%%

int yylex()
{
    char c;
    if (scanf("%c", &c) != EOF)
        return c;
    else
        return YYEOF;
}

void yyerror(const char *s) {
    fprintf(stderr, "%s\n", s);
}

int main() {
    return yyparse();
}

测试输入与输出对比 #

测试输入 #

illegal line
illegal line

未启用LAC的预期输出 #

syntax error, unexpected invalid token, expecting end of file or 'x'
error line
syntax error, unexpected invalid token, expecting end of file or 'x'
error line
program finished successfully!

启用LAC后的实际输出 #

syntax error, unexpected invalid token, expecting end of file or '!' or 'x'

调试日志信息 #

Starting parse
Entering state 0
Stack now 0
Reading a token
Next token is token invalid token ()
LAC: initial context established for invalid token
LAC: checking lookahead invalid token: Always Err
Constructing syntax error message
LAC: checking lookahead end of file: R2 G3 R4 G6 R1 G2 S5
LAC: checking lookahead '!': S1
LAC: checking lookahead '\n': R2 G3 R4 G6 Err
LAC: checking lookahead 'x': R2 G3 R4 G6 S8
syntax error, unexpected invalid token, expecting end of file or '!' or 'x'
Cleanup: discarding lookahead token invalid token ()
Stack now 0

疑问 #

推测问题可能源于解析器的Always Err决策，但对LAC工作原理了解有限。请问这是LAC的bug还是程序本身存在问题？能否修复该问题，或是LAC并不适用于当前场景？

已在Bison邮件列表提问，但暂未收到回复。

内容的提问来源于stack exchange，提问作者Piotr Siupa