I'd expect the compiler to allocate space for ErrorOr on the stack of the caller and pass an address to this space in a hidden parameter. In effect, the impleme

I'd expect the compiler to allocate space for ErrorOr on the stack of the caller and pass an address to this space in a hidden parameter. In effect, the implementation func0 would probably be typed a bit like this: void func0(llvm::ErrorOr<int> *__result) An advantage of passing a pointer to the error that you may be missing is that if you can pass this location all the way down, you can eliminate redundant copies / moves / assignments. This is harder when the return value and error value are combined in a single tagged union, so there will be a lot more data shuffling code all the way back up the stack.