[cfe-dev] Fwd: Review of transparent_union patch

[Douglas Gregor]

On Jan 27, 2009, at 12:30 AM, Eli Friedman wrote:

> On Mon, Jan 26, 2009 at 11:52 PM, Anders Johnsen <skabet at gmail.com>  
> wrote:
>> On Tue, Jan 27, 2009 at 5:21 AM, Eli Friedman  
>> <eli.friedman at gmail.com> wrote:
>>> On Mon, Jan 26, 2009 at 11:45 AM, Anders Johnsen  
>>> <skabet at gmail.com> wrote:
>>>>> Something like the following:
>>>>> struct x {int x; int* y;};
>>>>> typedef struct x a __attribute((transparent_union));
>>>>>
>>>>> This isn't valid, but it looks like your patch accepts it.
>>>>
>>>> I just tested this, and it looks like it worked:
>>>
>>> Oh, oops, make that "union x".
>>
>> gcc does't complain. I'm not sure if i should do anything? Where have
>> you read that it shouldn't allow such things?
>
> Hmm, I don't remember the details... if gcc accepts it, I suppose I
> must be mistaken.

GCC's attributes aren't handle consistently in the type system. Try  
this example:

   union x {int x; int* y;};
   typedef union x a __attribute((transparent_union));

   void foo(union x);
   void bar(a);

   void test(int i) {
     foo(i); // fails
     bar(i); // okay
   }

So, a typedef of "union x" has different semantics from "union x",  
despite the fact that those types should be identical. The GCC guys  
know about these problems; see the discussion at http://gcc.gnu.org/ml/gcc/2006-10/msg00318.html

I think that Clang should warn about the addition of "semantic"  
attributes (to steal Mark M.'s terminology) via typedefs, and say  
something about the typedef behaving differently from the original  
type due to the attribute. In C++ this is particularly important,  
since template instantiation needs to be based on the original type,  
even when one uses the typedef to name the instantiation (this is a  
particularly messy area in GCC).

However, it seems to me that this patch is ready to go in. Any other  
issues we need to address first?

	- Doug