yyabortlab bug report (and proposed fix)

[Wolfgang Spraul]

I'm working with bison 2.0 (release).
It generates the following code:

---
yyabortlab:
  yydestruct("Error: discarding lookahead", yytoken, &yylval);
  yychar = YYEMPTY;
  yyresult = 1;
  goto yyreturn;
---

One could argue why setting yychar = YYEMPTY is necessary, since the very next 
step is returning from the function, but more importantly, I have crashes in 
some cases where the lookahead item is destructed twice.
I could fix my crashes by changing the above fragment to:

---
yyabortlab:
  if (yychar != YYEOF && yychar != YYEMPTY)  
    yydestruct("Error: discarding lookahead", yytoken, &yylval);
  yychar = YYEMPTY;
  yyresult = 1;
  goto yyreturn;
---

I do not nearly oversee all bison cases, so I don't want to propose this as a 
'bug fix' (maybe it creates new bugs :-)). But my crash is fixed, and my 
understanding is that yylval and yytoken _depend_ on yychar (i.e. if yychar 
is YYEMPTY, yytoken and yylval are undefined). So we should check yychar 
before calling yydestruct() to free the yylval lookahead token.

Regards,
Wolfgang Spraul