Thanks! I’ve updated and sent [clang][nullability] allow _Nonnull etc on gsl::Pointer types by sam-mccall · Pull Request #82705 · llvm/llvm-project · GitHub based on our discussion here.
It implicitly adds the attribute to std::unique_ptr and friends. If we don’t have this consistently, I think we’d need some way for code to feature-test whether unique_ptr<int> _Nullable is going to work or be a compile error.