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A type constraint in Go is an interface that defines the permissible set of types that can be instantiated for a type parameter in a generic function or generic data structure. Introduced in Go 1.18, constraints expand the definition of interfaces from strictly representing a method set to representing a type set. If a type belongs to the type set defined by the constraint, it successfully satisfies that constraint.

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Syntax Visualization

Constraints are applied within square brackets [...] immediately following a function or type name, declaring the type parameter and its corresponding constraint. 
// Define a custom constraint
type Numeric interface {
    ~int | ~float64
}

// T is the type parameter, Numeric is the type constraint
func Process[T Numeric](input T) T {
    return input
}

// Multiple type parameters with different constraints
type Dictionary[K comparable, V any] struct {
    data map[K]V
}

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Interfaces as Type Sets

When an interface is used as a constraint, it dictates the exact types allowed. This is achieved using type elements. 
type SignedInteger interface {
    int | int8 | int16 | int32 | int64
}
In this context, SignedInteger is not checking for methods; it is defining a closed type set. Only the exact types listed can satisfy this constraint.

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The Union Operator (|)

The pipe | acts as a union operator within an interface definition. It combines multiple types into a single constraint. A type argument satisfies the constraint if it matches any of the types in the union. 
type Float interface {
    float32 | float64
}

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The Approximation Operator (~)

By default, a constraint requires a strict type match. If a constraint specifies int, a custom type defined as type MyInt int will fail the constraint check. The tilde ~ operator relaxes this by specifying the underlying type. 
type AnyString interface {
    ~string
}

type MyString string

// MyString satisfies AnyString because its underlying type is string.
Restriction: The ~ operator can only be applied to types whose underlying type is itself. This includes predeclared types (like int or string) and type literals (like []byte, map[string]int, or struct{ id int }). It cannot be applied to interfaces or type parameters.

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Inline Constraints

Constraints do not need to be explicitly declared as named interfaces. They can be defined anonymously directly within the type parameter list. 
func Add[T ~int | ~float64](a, b T) T {
    return a + b
}

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Predeclared Constraints

Go provides two built-in constraints that do not require explicit interface definitions:
  1. any: An alias for the empty interface interface{}. Its type set is infinite, meaning it imposes no restrictions. Any type satisfies any.
  2. comparable: A specialized, compiler-enforced constraint. Its type set consists of all types that support the equality operators == and !=. This includes booleans, numbers, strings, pointers, channels, and structs/arrays composed exclusively of comparable types. It explicitly excludes slices, maps, and functions.

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Combining Methods and Type Elements

A constraint can enforce both a specific type set and a method set simultaneously. A type argument must belong to the union of types and also implement all declared methods. 
type StringableInteger interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64
    String() string
}
Note: Interfaces that contain type elements (unions or approximations) can only be used as type constraints. They cannot be used as standard interface types for variable declarations.
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