pointers - Types of Go struct methods that satisfy an interface

Question

Given the following Go code example:

package main

import "fmt"

type greeter interface {
    hello()
    goodbye()
}

type tourGuide struct {
    name string
}

func (t tourGuide) hello() {
    fmt.Println("Hello", t.name)
}

func (t *tourGuide) goodbye() {
    fmt.Println("Goodbye", t.name)
}

func main() {
    var t1 tourGuide = tourGuide{"James"}
    t1.hello()   // Hello James
    t1.goodbye() // Goodbye James (same as (&t1).goodbye())

    var t2 *tourGuide = &tourGuide{"Smith"}
    t2.hello()   // Hello Smith
    t2.goodbye() // Goodbye Smith (same as (*t2).hello())

    // illegal: t1 is not assignable to g1 (why?)
    // var g1 greeter = t1

    var g2 greeter = t2
    g2.hello()   // Hello Smith
    g2.goodbye() // Goodbye Smith
}

I'm able to call the two methods of the struct tourGuide using either a variable of type tourGuide t1 or a pointer to tourGuide t2. In other words, I can call a method with T receiver using a variable of type T or *T. Similarly, I can call a method with *T receiver using a variable of type T (if T is addressable) or *T. I understand that the compiler handles the differences here (see my comments in the code).

However, things change when we are implementing interfaces. In the above code, a variable of type greeter interface is assignable from a pointer to tourGuide but not from a tourGuide.

Can anyone tell me why this is the case? Why am I able to call t1.hello() and t1.goodbye() but somehow t1 is not enough for the interface greeter?

Answer 1

Waitting for answers