Go でポインター レシーバー メソッドを使用して単純型を変更すると期待どおりに機能しないのはなぜですか?

package main

import "fmt"

type FooInt int

func (fi *FooInt) FromString(i string) {
    num, _ := strconv.Atoi(i)
    tmp := FooInt(num)
    fi = &tmp // Attempting to modify the pointer
}

func main() {
    var fi *FooInt
    fi.FromString("5")
    fmt.Println(fi) // Prints nil
}

func (fi *FooInt) FromString(i string) *FooInt {
    num, _ := strconv.Atoi(i)
    tmp := FooInt(num)
    return &tmp
}

func main() {
    var fi *FooInt
    fi = fi.FromString("5")
    fmt.Println(fi) // Prints the address of the modified pointer
}

func (fi *FooInt) FromString(i string) {
    if fi == nil {
        fi = new(FooInt) // Initialize fi if it's nil
    }
    num, _ := strconv.Atoi(i)
    *fi = FooInt(num)
}

func main() {
    var fi *FooInt
    fi.FromString("5")
    fmt.Println(fi) // Prints the address of the modified pointer
}