[C PATCH] warn for empty struct -Wc++-compat

[Prathamesh Kulkarni]

On Tue, Nov 11, 2014 at 4:30 PM, Marek Polacek <polacek@redhat.com> wrote:
> On Tue, Nov 11, 2014 at 04:45:46AM +0530, Prathamesh Kulkarni wrote:
>> Index: gcc/c/c-decl.c
>> ===================================================================
>> --- gcc/c/c-decl.c    (revision 217287)
>> +++ gcc/c/c-decl.c    (working copy)
>> @@ -606,6 +606,8 @@
>>    /* If warn_cxx_compat, a list of typedef names used when defining
>>       fields in this struct.  */
>>    vec<tree> typedefs_seen;
>> +  /* code to distinguish between struct/union */
>> +  enum tree_code code;
>
> I don't think this is desirable, you might just pass T down from
> finish_struct to warn_cxx_compat_finish_struct.
>
>> @@ -7506,12 +7509,19 @@
>>  /* Finish up struct info used by -Wc++-compat.  */
>>
>>  static void
>> -warn_cxx_compat_finish_struct (tree fieldlist)
>> +warn_cxx_compat_finish_struct (tree fieldlist, location_t record_loc)
>>  {
>>    unsigned int ix;
>>    tree x;
>>    struct c_binding *b;
>>
>> +  if (fieldlist == NULL_TREE)
>> +    {
>> +      warning_at (record_loc, OPT_Wc___compat,
>> +               "empty %s has size 0 in C, 1 in C++",
>> +               (struct_parse_info->code == RECORD_TYPE) ? "struct" : "union");
>> +    }
>> +
>
> I think this won't work well wrt translations, so you need to have
> an if here.  See the pedwarns at the beginning of finish_struct.
>
>> Index: gcc/testsuite/gcc.dg/Wcxx-compat-22.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/Wcxx-compat-22.c     (revision 0)
>> +++ gcc/testsuite/gcc.dg/Wcxx-compat-22.c     (working copy)
>> @@ -0,0 +1,4 @@
>> +/* { dg-do compile } */
>> +/* { dg-options "-Wc++-compat" } */
>> +struct A {}; /* { dg-warning "empty struct has size 0 in C" } */
>> +union B {}; /* { dg-warning "empty union has size 0 in C" } */
>
> Please also test an empty struct in a struct.
I tried the following:
struct A { struct B {}; int x; } /* { dg-warning "empty struct has
size 0 in C" } */
                                           /* { dg-bogus "delcaration
does not declare anything" } */
but it fails in excess errors.
How do I resolve this ?

Is the rest okay ?
[gcc/c]
  * c-decl.c (warn_cxx_compat_finish_struct): New parameters code, record_loc.
        Warn for empty struct.
    (finish_struct): Pass TREE_CODE (t) and loc to
warn_cxx_compat_finish_struct.

[gcc/testsuite/gcc.dg]
  * Wcxx-compat-22.c: New test-case.

Thank you,
Prathamesh
>
> Thanks,
>
>         Marek
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