[clang] [Clang][CodeGen] Bail out on constexpr unknown values in ConstantEmitter (PR #127525)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm worried this is just scratching the surface... it looks like there's a whole class of issues involving constexpr-unknown leaking out of constant evaluation, and this is just addressing one specific case.  If we can't take https://github.com/llvm/llvm-project/pull/128409 on the branch, we should just disable the feature and give it more time to bake on main.

https://github.com/llvm/llvm-project/pull/127525