Can I return a union type on outputFields?

[noghartt]

Is there a way that I can return a union type on outputFields? I want to do something like it:

mutation userRegister($input: UserRegisterInput!) {
  userRegister(input: $input) {
    ... on UserRegisterResultSuccess {
      user {
        id
      }
    }
    ... on Error {
      message
    }
  }
}

I would like to know if there's a way to do it on outputFields of a mutation.