std::experimental::nullopt_t - cppreference.com
From cppreference.com
std::experimental::nullopt_t is an empty class type used to indicate optional type with uninitialized state. In particular, std::experimental::optional has a constructor with nullopt_t as a single argument, which creates an optional that does not contain a value.
std::experimental::nullopt_t must be a LiteralType and cannot have a default constructor.
It must have a constexpr constructor that takes some implementation-defined literal type.
Notes
nullopt_t is not DefaultConstructible to support both op = {}; and op = nullopt; as the syntax for disengaging an optional object.
A possible implementation of this class is
struct nullopt_t { constexpr nullopt_t(int) {} };