environment variable when running the go tool: set it to 1 to enable
the use of cgo, and to 0 to disable it. The go tool will set the
build constraint "cgo" if cgo is enabled. The special import "C"
implies the "cgo" build constraint, as though the file also said
"//go:build cgo".  Therefore, if cgo is disabled, files that import
"C" will not be built by the go tool. (For more about build constraints

values or variables, or components of other, non-interface types.
</p>

<h4 id="Satisfying_a_type_constraint">Satisfying a type constraint</h4>

<p>
A type argument <code>T</code><i> satisfies</i> a type constraint <code>C</code>
if <code>T</code> is an element of the type set defined by <code>C</code>; i.e.,

// (booleans, numbers, strings, pointers, channels, arrays of comparable types,
// structs whose fields are all comparable types).
// The comparable interface may only be used as a type parameter constraint,
// not as the type of a variable.
type comparable interface{ comparable }

// iota is a predeclared identifier representing the untyped integer ordinal

FieldName     = identifier .
Element       = Expression | LiteralValue .
</pre>

<p>
The LiteralType's underlying type must be a struct, array, slice, or map type
(the grammar enforces this constraint except when the type is given
as a TypeName).
The types of the elements and keys must be <a href="#Assignability">assignable</a>
to the respective field, element, and key types of the literal type;

	n := typ.NumMethods()
	list := make([]string, n)
	for i := range list {
		list[i] = typ.Method(i).Name()
	}
	sort.Strings(list)
	return list
}

// sortedEmbeddeds returns constraint types embedded in an
// interface. It does not include embedded interface types or methods.
func (w *Walker) sortedEmbeddeds(typ *types.Interface) []string {
	n := typ.NumEmbeddeds()
	list := make([]string, 0, n)