custom attribute proposal (yeah, another one)

[Steven Schveighoffer]

On Sat, 07 Apr 2012 12:48:00 -0400, Jacob Carlborg <doob at me.com> wrote:

> On 2012-04-07 05:29, Kapps wrote:
>
>> I slightly prefer this function method over the struct method because:
>> 1) No need to generate a custom struct for everything. Plenty of things
>> are just a true or false, or a string. Saves a little bit of TypeInfo
>> generation.
>
> But you still need to create a function.

functions are easier for the linker to deal with.  The main point here is,  
no TypeInfo needed.

>
>> 2) The more important one: The possibility to eventually include an
>> alias template parameter. This allows things like looking up whether the
>> symbol with the attribute has other attributes applied, or determining
>> type. This allows things like constraints, and can be a nice benefit.
>
> This can't be done for structs?

IFTI.  It possibly can be added to struct ctors (I argue it should be),  
but is not today.

I think the struct approach is fine for some attributes, and I think it  
should be doable to @attribute either functions or structs.  I just want  
the most generic, basic feature possible.  I think Timon has the best idea  
that any callable CTFE symbol should be able to be an attribute.

At this point it has become a "structs are a good solution, why not also  
allow functions?" argument.

-Steve