How does equality work for interface values in Go?

type Animal interface{ Sound() string }
type Dog struct{ Name string }
func (d Dog) Sound() string { return "Woof" }

var a1 Animal = Dog{Name: "Rex"}
var a2 Animal = Dog{Name: "Rex"}
var a3 Animal = Dog{Name: "Buddy"}

fmt.Println(a1 == a2) // true  — same type, same value (Dog is comparable)
fmt.Println(a1 == a3) // false — same type, different Name

// Nil interface equality
var a4 Animal
fmt.Println(a4 == nil)  // true  — both words nil
fmt.Println(a1 == nil)  // false — a1 holds a Dog

// Nil pointer in interface — NOT equal to nil
var d *Dog
var a5 Animal = d
fmt.Println(a5 == nil)  // FALSE — type word is non-nil (*Dog)

// Comparing interfaces with non-comparable concrete types panics!
type BadAnimal interface{ Sound() string }
type SliceDog struct{ Tags []string }
func (s SliceDog) Sound() string { return "Woof" }

var b1 BadAnimal = SliceDog{Tags: []string{"cute"}}
var b2 BadAnimal = SliceDog{Tags: []string{"cute"}}
// fmt.Println(b1 == b2) // PANIC: runtime error: comparing uncomparable type SliceDog

// Safe comparison with reflect.DeepEqual
fmt.Println(reflect.DeepEqual(b1, b2)) // true — but slower