How do generics relate to interfaces in Go 1.18+ and what are type constraints?

Go 1.18 extended the interface syntax so interfaces can serve as type constraints for generic functions and types. A constraint specifies which types a type parameter can be. Constraints are just interfaces — but now interfaces can include concrete type lists in addition to method sets.

// Constraint: any type with a < operator (from golang.org/x/exp/constraints)
type Ordered interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64 |
    ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 |
    ~float32 | ~float64 | ~string
}

// Generic function constrained by Ordered
func Min[T Ordered](a, b T) T {
    if a < b { return a }
    return b
}

fmt.Println(Min(3, 5))         // 3  — T inferred as int
fmt.Println(Min(3.14, 2.71))   // 2.71 — T inferred as float64
fmt.Println(Min("foo", "bar")) // bar — T inferred as string

// The ~ (tilde) prefix means: any type whose underlying type is int
type Celsius float64
Min(Celsius(20), Celsius(30)) // works because ~float64 matches Celsius

// Interface as both constraint and regular interface
type Stringer interface { String() string }

func Describe[T Stringer](v T) string {
    return ">> " + v.String()
}

// Same interface used as constraint (generic) and variable type (runtime)
var s Stringer = Color(Red)      // runtime interface variable
result := Describe(Color(Red))   // compile-time generic instantiation

// any is also a valid constraint — equivalent to interface{}
func Keys[K comparable, V any](m map[K]V) []K {
    keys := make([]K, 0, len(m))
    for k := range m { keys = append(keys, k) }
    return keys
}

The key distinction: when an interface with a type list (e.g., ~int | ~float64) is used as a regular variable type, only method calls are allowed — the type list restricts which types can satisfy it. When used as a type constraint, the full type list is available to the compiler for optimisation and operator permissions (like <).

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