[PATCH 1/2 v2] Introduce __builtin_is_virtual_base_of

[Jason Merrill]

On 7/30/24 6:49 PM, Giuseppe D'Angelo wrote:
> On 29/07/2024 22:53, Giuseppe D'Angelo wrote:
>> Hi,
>>
>> The attached patch is a stab at adding the necessary compiler builtin to
>> support std::is_virtual_base_of (P2985R0, approved for C++26). The name
>> of the builtin matches the one just merged into clang:
>>
>> https://github.com/llvm/llvm-project/issues/98310
>>
>> The next patch will add the libstdc++ bits.
> 
> Hello,
> 
> This is a new revision of the same patch, this time with a DCO.

> +  ba_require_virtual = 1 << 3 /* Require a virtual base */

Why change lookup rather than just call lookup_base and then 
binfo_from_vbase in trait_expr_value?

Jason