why you have to avoid the assignment of the dysfunctional value to the interface [golang]

main.go

// https://play.golang.org/p/1hGHFwjlwud
package main

import (
	"bytes"
	"fmt"
	"io"
)

func main() {
	var w1 *bytes.Buffer
	var w2 io.Writer

	fmt.Printf("w1: %T\n", w1) // *bytes.Buffer
	fmt.Printf("w2: %T\n", w2) // <nil>

	if w2 != nil {
		fmt.Println(" ... doing something with w2 - it panics here")
	}

	var w3 *bytes.Buffer = nil
	var w4 io.Writer = nil

	fmt.Printf("w3: %T\n", w3) // *bytes.Buffer
	fmt.Printf("w4: %T\n", w4) // <nil>

	if w4 != nil {
		fmt.Println(" ... doing something with w4 - it panics here")
	}
	
	var w5 *bytes.Buffer
	var w6 io.Writer = w5

	fmt.Printf("w5: %T\n", w5) // *bytes.Buffer
	fmt.Printf("w6: %T\n", w6) // *bytes.Buffer
	
	if w6 != nil {
		fmt.Println(" ... doing something with w6 - it panics here")
	}
}

output.txt

w1: *bytes.Buffer
w2: <nil>
w3: *bytes.Buffer
w4: <nil>
w5: *bytes.Buffer
w6: *bytes.Buffer
 ... doing something with w6 - it panics here