[clang] [Clang][Sema] Diagnose unexpanded packs in the template argument lists of function template specializations (PR #76677)

[Nico Weber via cfe-commits]

nico wrote:

This broke check-clang-tools: http://45.33.8.238/linux/127018/step_8.txt

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/76677