> I would think that the returned value would be: `(nil, return-value-of-Decode-call)`.

`user` is typed as a struct, so it's always going to be a struct in the output, it can't be nil (it would have to be `*User`). And Decoder.Decode mutates the parameter in place. Named return values essentially create locals for you. And since the function does not use naked returns, it's essentially saving space (and adding some documentation in some cases though here the value is nil) for this:

    func fetchUser(id int) (User, error) {
        var user User
        var err Error

        resp, err := http.Get(fmt.Sprintf("https://api.example.com/users/%d", id))
        if err != nil {
            return user, err
        }
        defer resp.Body.Close()
        return user, json.NewDecoder(resp.Body).Decode(&user)
    }

However Go's named return values are definitely weird and spooky:

    func foo() (i int) {
     defer func() {
      i = 2
     }()
     return 1
    }