Simple demonstration of how nil pointers are not nil interfaces

README.md

Plauground

References

• os.PathError
• os.PathError.Error
• Go Spec - Assignability

nil.go

package main

import (
	"fmt"
	"os"
)

func main() {
	// p is not an interface. it is a pointer to a struct https://golang.org/pkg/os/#PathError
	var p *os.PathError
	fmt.Printf("%5v %p %[3]T %[3]v\n", p == nil, &p, p)

	// err is an interface
	var err error
	fmt.Printf("%5v %p %[3]T %[3]v\n", err == nil, &err, err)
	// Assinging p to error is effectively like a type-cast and works since
	// a `*os.PathEror` is assignable to `error` because
	//   T is an interface type and x implements T.
	//  `error` is an interface type and `*os.PathEror` implements `error`. https://golang.org/pkg/os/#PathError.Error
	// https://golang.org/ref/spec#Assignability:~:text=T%20is%20an%20interface%20type%20and%20x%20implements%20T.
	err = p
	fmt.Printf("%5v %p %[3]T %[3]v\n", err == nil, &err, err)

	// This may make it more clear.
	// Ignoring that error is a specific type of interface,
	// this otherwise works exactly the same.
	// Since everything is assignable to the empty interface.
	var empty interface{}
	fmt.Printf("%5v %p %[3]T %[3]v\n", empty == nil, &empty, empty)
	empty = p
	fmt.Printf("%5v %p %[3]T %[3]v\n", empty == nil, &empty, empty)
}