libstdc++/api/a12980.html

template<typename _Out, typename _Tp>

concept std::indirectly_writable =  requires(_Out&& __o, _Tp&& __t)

      {

        *__o = std::forward<_Tp>(__t);

        *std::forward<_Out>(__o) = std::forward<_Tp>(__t);

        const_cast<const iter_reference_t<_Out>&&>(*__o)

          = std::forward<_Tp>(__t);

        const_cast<const iter_reference_t<_Out>&&>(*std::forward<_Out>(__o))

          = std::forward<_Tp>(__t);

      }

std::forward
constexpr _Tp && forward(typename std::remove_reference< _Tp >::type &__t) noexcept
Forward an lvalue.
Definition move.h:72

std::indirectly_writable
Requirements for writing a value into an iterator's referenced object.
Definition iterator_concepts.h:584