[clang] [libcxx] Reland: [clang] unified CWG2398 and P0522 changes; finishes implementation of P3310 (PR #124137)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> Hi @mizvekov
> 
> I believe this change is impacting our downstream compiler. Here is a minimal example:
> 
> ```
> #include <utility>
> #include <cstddef>
> namespace  {
> 
>   template <typename INDEX_SEQUENCE>
>   class ForEach; // intentionally undefined                                                                                                                                                            
> 
>   template <size_t... INDICES, template <typename, size_t...> class INDEX_CONTAINER>
>   class ForEach<INDEX_CONTAINER<size_t, INDICES...>> {};
> 
> } //namespace                                                                                                                                                                                          
> 
> static void go()
> {
>   ForEach<std::make_index_sequence<42>> X;
> }
> ```
> 
> It was accepted by clang 19.1.0 and gcc but not the clang trunk: https://godbolt.org/z/hdYWq86Ke
> 
> I'm not an expert, could you explain to me why `ForEach<std::make_index_sequence<42>>` does not specialize this template ` class ForEach<INDEX_CONTAINER<size_t, INDICES...>> {};`?

@mizvekov, can you take a look at this?

https://github.com/llvm/llvm-project/pull/124137