std::experimental::ranges::IndirectRelation

template < class F, class I1, class I2 = I1 >

concept bool IndirectRelation =
  Readable<I1> && Readable<I2> &&
  CopyConstructible<F> &&
  Relation<F&, ranges::value_type_t<I1>&, ranges::value_type_t<I2>&> &&
  Relation<F&, ranges::value_type_t<I1>&, ranges::reference_t<I2>> &&
  Relation<F&, ranges::reference_t<I1>, ranges::value_type_t<I2>&> &&
  Relation<F&, ranges::reference_t<I1>, ranges::reference_t<I2>> &&

  Relation<F&, ranges::iter_common_reference_t<I1>, ranges::iter_common_reference_t<I2>>;