Attributes on structs

[Jason Merrill <jason at redhat dot com>]

There are several outstanding bugs (29436, 28560, 28834, possibly 
others) having to do with applying attributes to uses of structs in 
typedefs or other places other than the point of definition.  The 
problem with doing this is that the attribute code makes a new 
TYPE_MAIN_VARIANT, so we basically get a new type which shares the name 
and members of the original type.  This causes trouble for the C++ type 
system and other parts of the compiler that assume that all variants of 
a class with a particular name have the same TYPE_MAIN_VARIANT.

I notice that the GCC manual only says that type attributes can be 
applied in a typedef or type definition, but in fact the compiler allows 
them to be applied anywhere.

Sometimes the attributes are intended to create new types (vector), but 
usually not; the modified types are intended to work like the base type. 
 Sometimes the attribute has important implications for code generation 
(aligned), sometimes it doesn't (unused).

For the cases that have important implications for code generation, we 
need the TYPE_MAIN_VARIANT trick because a lot of places in the compiler 
assume that they can just take TYPE_MAIN_VARIANT to strip cv-quals, and 
we don't want that to discard important attributes.

One solution to this issue would be to simply disallow attributes on 
structs after the definition.  Failing that, we need to define how they 
interact with the type system.  Opinions?

Jason