[cfe-dev] [PATCH] Allow unnamed structure or union fields within structs/unions in gnu99 mode

[Pierre d'Herbemont]

Eli,

On Nov 9, 2008, at 8:01 PM, Eli Friedman wrote:

> On Sun, Nov 9, 2008 at 8:53 AM, Pierre d'Herbemont <pdherbemont at free.fr 
> > wrote:
>> Hi,
>>
>> This patch enables unnamed structure or union fields withing  
>> structs/unions
>> when -std=gnu99 is used. Cf [1].
>>
>> Note, as stated in [1], struct { int a; union { int a; } } is  
>> discouraged,
>> but won't produce any error nor warning.
>> It is unclear for me that we really want to add anonymous structure  
>> support
>> in RecordDecl::getMember, as implementation could vary given the  
>> language
>> standard. Yet, not implementing here could lead to easy and  
>> unoticeable
>> mistakes if the caller doesn't implement anonymous structure  
>> resolving.
>>
>> I'll be happy to ear your thoughts...
>
> It looks generally fine to me.  It's probably a good idea for someone
> else to review it, though.
>
> A couple of small comments:
>
> 1. Do we want to support this extension in gnu89 mode?

Right.

The docs does not state that explicitly, yet it is supported.

$ gcc -c -std=gnu89 test/Sema/unnamed-struct-in-struct.c
$

Doug suggested to do that always unless NoExtensions is specify this  
is what I've done... Yet, -std=c99 doesn't enable NoExtensions... Weird.

> 2. Does this correctly deal with "struct foo {int a,b;}; struct bar
> {int a; struct foo;};"

Well, nice catch as well:

$ gcc -c test-2.c
test-2.c:2: warning: declaration does not declare anything
test-2.c: In function 'dummy':
test-2.c:8: error: 'struct bar' has no member named 'b'
$

Previous version enables such an anonymous struct usage. Attached  
patch filters this out as well.

> "struct bar {int a; struct foo {int a,b;};};"?

$ gcc -c test-3.c
test-3.c:1: warning: declaration does not declare anything
test-3.c: In function 'dummy':
test-3.c:6: error: 'struct bar' has no member named 'b'
$

Previous version enables such an anonymous struct usage. Attached  
patch filters this out as well.

> 3. For "struct { int a; union { int a; } }", can we produce an error?
> The gcc docs seem to imply that's it's illegal, and it's usually
> better to produce an error where we can.

Actually current gcc does not produce error as stated in the doc. Yet,  
I agree that this would be way better to output error.

Patch is attached to next email in this thread.

Pierre.