Re: RFC: generate the default semantic action

[Akim Demaille]

> Le 16 oct. 2018 à 07:13, Akim Demaille <address@hidden> a écrit :
> 
> Hi Rici,
> 
>> This patch won't affect programs which rely on this feature, as far as I
>> can see. But I wanted to highlight the issue because the road that follows
>> this patch might inadvertently affect existing programs which count on the
>> default action to be a copy of the *entire* semantic value.
> 
> No, I don’t think it changes anything, indeed.  Even type checking
> is run before adding the explicit default action.  Your example
> runs fine.

Scratch that, my experiments were broken.  Like C’s handling of arrays.

This language treats arrays in a special way, forbidding assignments
between arrays, but accepting them happily if the arrays are members
of structs and you assign the structs.

Consider this:

%{
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

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

typedef char string[256];
%}

%union {
  string str;
};

%token <str> WORD
%type <str> word
%%

prog: %empty
   | prog word        { printf("%s\n", $2); }

word:   WORD

%%

int yylex(void) {
  static int c = 0;
  int val = c++;
  if (val < 4)
    {
      sprintf(yylval.str, "%d", val);
      return WORD;
    }
  else
    return YYEOF;
}

int main(int argc, char** argv) { return yyparse(); }


The old bison is happy with it, and it generates valid C code.

$ /opt/local/bin/bison /tmp/foo.y && gcc-mp-7 foo.tab.c && ./a.out
0
1
2
3

The ‘new’ bison is happy with it, but generates invalid code.

$ ./_build/8s/tests/bison /tmp/foo.y && gcc-mp-7 foo.tab.c
/tmp/foo.y: In function 'yyparse':
/tmp/foo.y:24:19: error: assignment to expression with array type
 word:   WORD
                   ^

This example is dubious, is the user really likely to expect

  word: WORD

to behave properly without strncpy?  In practice, it does, but…


So I think I will (for a start?) restrict this change to only
C++ with variants.

WDYT?