[clang] [clang][Sema] fix crash on __type_pack_element with dependent packs (GH180307) (PR #180407)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

@cor3ntin would you also be able to track this - I'm not sure my current work priorities means I'll be able to respond at a reason rate

https://github.com/llvm/llvm-project/pull/180407