The optimized ABI doesn’t depend on there being one error type for everything. Ideally it should be usable for any function parameter or return of a type that

The optimized ABI doesn’t depend on there being one error type for everything. Ideally it should be usable for any function parameter or return of a type that can handle being moved with memcpy - not just errors. And indeed, in the current Clang implementation you can stick [[trivial_abi]] on any class.