How do type aliases and type definitions differ in relation to interface satisfaction in Go?

Go distinguishes between type definitions (type MyInt int) and type aliases (type MyInt = int). They behave very differently when it comes to interface satisfaction and method sets.

// TYPE DEFINITION — creates a new, distinct type
type Celsius float64
type Fahrenheit float64

// Celsius and float64 are different types — no implicit conversion
var c Celsius = 100.0
// var f float64 = c  // COMPILE ERROR: cannot use Celsius as float64
var f float64 = float64(c) // explicit conversion required

// Methods can be defined on type-defined types
func (c Celsius) ToFahrenheit() Fahrenheit { return Fahrenheit(c*9/5 + 32) }

// TYPE ALIAS — another name for the same type
type MyFloat64 = float64   // alias: MyFloat64 IS float64
var x MyFloat64 = 3.14
var y float64 = x          // OK — same type
// Cannot define methods on alias types (not in the same package as the original)

// Interface satisfaction:
type Stringer interface { String() string }

// Only defined types can have methods (and thus satisfy interfaces via those methods)
func (c Celsius) String() string { return fmt.Sprintf("%.1f°C", float64(c)) }
var s Stringer = Celsius(100) // OK — Celsius has String()

// Named function types (common for interface satisfaction)
type HandlerFunc func(string) error
func (f HandlerFunc) Handle(s string) error { return f(s) }  // method on func type

type Handler interface { Handle(string) error }
var h Handler = HandlerFunc(func(s string) error { return nil })

Key rule: you can define methods on a type-defined type (creating a new type gives it its own method set). You cannot add methods on an alias type (it's the same type — adding methods would affect the original type's package). This is why type MyInt int can satisfy a new interface that int cannot — you can add methods to MyInt.

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