[clang] [Clang] Implement C++26’s P2893R3 ‘Variadic friends’ (PR #101448)

[via cfe-commits]

Sirraide wrote:

> I would recommend creating an issue and see how ewg handles the new information before reaping out a lot of code.

Hmm, I mean, it looks like they’ve already thought about the consequences though, or am I missing something here, because the case that we currently support and which this ends up disallowing is explicitly listed as disallowed in the core issue:
> The proposed resolution above disallows a few examples from paper [P2893R3](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2893r3.html) (Variadic friends), such as
> ```c++
>  template<class... Ts>
>  struct VS {
>    template<class U>
>    friend class C<Ts>::Nested...; // now ill-formed
>  };
> ```

> https://lists.isocpp.org/core/2024/08/16236.php

Ah, that I don’t appear to have access to unfortunately.

https://github.com/llvm/llvm-project/pull/101448