What is the difference between a defined type (type A B) and an alias (type A = B)?

type Celsius float64 creates a new, distinct type. It has the same underlying type, but it doesn't inherit the methods of the original named type, and you need explicit conversions to mix it with the original. type A = B is an alias: another name for exactly the same type, with the same methods and full interchangeability. Aliases were added to support gradual refactoring across packages. Since Go 1.24, aliases can have type parameters.

type MyMutex sync.Mutex
var m MyMutex
// m.Lock()                 // compile error: MyMutex has no method Lock

type Locker = sync.Mutex
var l Locker
l.Lock()                    // OK: same type as sync.Mutex

type Celsius float64
type Fahrenheit float64
c := Celsius(100)
// f := Fahrenheit(0) + c   // compile error: mismatched types
f := Fahrenheit(c*9/5 + 32) // explicit conversion

// Methods of an EMBEDDED field survive in a defined struct type:
type SafeMap struct {
	sync.Mutex
	m map[string]int
}
type Other SafeMap // Other still has Lock/Unlock (promoted from the embedded field),
                   // but no methods declared directly on SafeMap

Why use defined types: type safety for units and IDs, attaching methods (for example type Handler func(...) with a ServeHTTP method), and building enums. byte and rune are built-in aliases for uint8 and int32, and any is an alias for interface{}.