Let’s look at some interface examples from the spec:

General interfaces

…

// An interface representing only the type int.
interface {
	int
}

// An interface representing all types with underlying type int.
interface {
	~int
}

// An interface representing all types with underlying type int that implement the String method.
interface {
	~int
	String() string
}

// An interface representing an empty type set: there is no type that is both an int and a string.
interface {
	int
	string
}

These examples should be pretty self-documenting. But let’s consider an imaginary example of the third case, as that may be the most interesting.

Let’s consider the HTTP status constants in the net/http package. Although these are untyped constants, we could easily imagine a custom type for these, perhaps defined in our own package:

type HTTPStatus int

const (
  StatusContinue           = HTTPStatus(100) // RFC 9110, 15.2.1
  StatusSwitchingProtocols = HTTPStatus(101) // RFC 9110, 15.2.2
  StatusProcessing         = HTTPStatus(102) // RFC 2518, 10.1
  StatusEarlyHints         = HTTPStatus(103) // RFC 8297

  StatusOK                   = HTTPStatus(200) // RFC 9110, 15.3.1
  StatusCreated              = HTTPStatus(201) // RFC 9110, 15.3.2
  /* etc ... */
)

// String() returns a text for the HTTP status code. It returns the empty
// string if the code is unknown.
func (s HTTPStatus) String() string {
  return http.StatusText(s)
}

Quotes from The Go Programming Language Specification Version of December 15, 2022