go - Golang type aliasing (type SampleType otherType !== otherType)?

I am building a simple router for which the code is below.

I have a callback that takes type Res as a parameter. Initially I was under the impression that type Res http.ResponseWriter would simply mean that Res is an alias for http.ResponseWriter, as passing http.ResponseWriter into the callback with a Res param worked fine.

I later decided I wanted to wrap my own functionality around the response provided to the callback, and so changed the type Res http.ResponseWriter to type Res response.Response - response being my own package (please look at what is commented out in the source below). After doing so I get "Cannot use newResponse (type response.Response) as type Res in argument to urlCallback"

I looked at the http.ResponseWriter source code and noticed it is an interface whereas my response.Response is a struct, is this where the problem lies?

Does setting a type as another type does not necessarily alias it? And if not, then why did passing in http.ResponseWriter for type Res work initially?

package Router

import (
    "fmt"
    "net/http"
    "net/url"
    "log"
    // "./response"
)


// type Res response.Response
type Res http.ResponseWriter
type Req *http.Request

type RouteMap map[string]func(Res, Req) 
type MethodMap map[string]RouteMap

type Router struct {
    Methods MethodMap

}

func (router *Router) Get(urlString string, callback func(Res, Req)) {
    parsedUrl, err := url.Parse(urlString)

    if(err != nil) {
        panic(err)
    }

    router.Methods["GET"][parsedUrl.Path] = callback

}

func (router *Router) initMaps() {
    router.Methods = MethodMap{}
    router.Methods["GET"] = RouteMap{}
}

func (router Router) determineHandler(w http.ResponseWriter, r *http.Request) {
    methodMap := router.Methods[r.Method]
    urlCallback := methodMap[r.URL.Path]


    // newResponse := response.NewResponse(w)

    if(urlCallback != nil) {
        // urlCallback(newResponse, r)
        urlCallback(w, r)
    }
}

func (router Router) Serve(host string, port string) {

    fullHost := host + ":" + port

    fmt.Println("Router is now serving to:" + fullHost)
    http.HandleFunc("/", router.determineHandler)

    err := http.ListenAndServe(fullHost, nil)

    if err == nil {
        fmt.Println("Router is now serving to:" + fullHost)
    } else {

        fmt.Println("An error occurred")

        log.Fatal(err)
    }
}

The commented out code describes the issue im having/what im trying to do

Answer

Doing

type Res response.Response

you define new type, which is not interchangeable with original

to convert you should do

newResponse := Res(response.NewResponse(w))

when you do

type Res http.ResponseWriter

your type Res is an interface and http.ResponseWriter implement it, so can be used as arg in func where Res wanted