The optional_should_convert concept.  
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#include <range/v3/utility/optional.hpp>
template<typename U, typename T>
        !(
            constructible_from<T, optional<U> &       > ||
            constructible_from<T, optional<U> &&      > ||
            constructible_from<T, optional<U> const & > ||
            constructible_from<T, optional<U> const &&> ||
            convertible_to<optional<U> &,        T> ||
            convertible_to<optional<U> &&,       T> ||
            convertible_to<optional<U> const &,  T> ||
            convertible_to<optional<U> const &&, T>
        )
The optional_should_convert concept.
Definition: optional.hpp:476
The optional_should_convert concept.