[clang] [Clang][Sema] Reject undeduced static data members without initializers (#173349) (PR #174281)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

> > While we are here, could you handle the case that `u` has an initializer?
> 
> It is already handled 

Oh, I see. We already emit diagnostics for `static E u2 = 0;` during template instantiation. With this patch, we emit diagnostics for `static E u;` before template instantiation. It would be better to handle the former case before template instantiation.

https://github.com/llvm/llvm-project/pull/174281