We could in principle support such pack expansion, using techniques similar to
what we do for pack expansion of lambdas, but it's not clear it's worthwhile.
For now at least, cleanly reject these cases rather than crashing.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@324160 91177308-0d34-0410-b5e6-96231b3b80d8
Richard Smith