const value_type &assume_value() const & noexcept
Narrow contract const lvalue reference observer of any value present. NoValuePolicy::narrow_value_check() is first invoked, then the reference to the value is returned. The convention is that hard undefined behaviour occurs if no value is actually present, however NoValuePolicy::narrow_value_check() can do something to avoid that.
Note that if value_type is void, only a const overload returning void is present.
Requires: Always available.
Complexity: Depends on NoValuePolicy::narrow_value_check().
Guarantees: An exception is never thrown.



