From the spec:
An embedded type must be specified as a type name T or as a pointer to a non-interface type name *T, and T itself may not be a pointer type.
You see that it mentions a "type name".
Named types are specified by a (possibly qualified) type name; unnamed types are specified using a type literal, which composes a new type from existing types.
In other words, a map or slice may not be anonymous unless they are defined as a named type. For example:
type MyMap map[string]string
type customMap struct{
MyMap
ordered []string
}
However, even if you embed MyMap or a slice type, you would still not be able to index customMap. Only fields and methods may be "promoted" when you embed. For everything else they act as just another field. In the above example, MyMap doesn't have any fields or methods and therefore is equivalent to:
type customMap struct{
MyMap MyMap
ordered []string
}