[iterator.cust.swap]

Let iter-exchange-move be the exposition-only function template:

template<class X, class Y>
  constexpr iter_value_t<X> iter-exchange-move(X&& x, Y&& y)
    noexcept(noexcept(iter_value_t<X>(iter_move(x)) &&
             noexcept(*x = iter_move(y));

Effects: Equivalent to: iter_value_t<X> old_value(iter_move(x)); *x = iter_move(y); return old_value;

The expression ranges::iter_swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to:

(4.1)
(void)iter_swap(E1, E2), if either E1 or E2 has class or enumeration type and iter_swap(E1, E2) is a well-formed expression with overload resolution performed in a context that includes the declaration template<class I1, class I2> void iter_swap(I1, I2) = delete; and does not include a declaration of ranges::iter_swap.

If the function selected by overload resolution does not exchange the values denoted by E1 and E2, the program is ill-formed, no diagnostic required.

[Note 1:
This precludes calling unconstrained std::iter_swap.

When the deleted overload is viable, program-defined overloads need to be more specialized ([temp.func.order]) to be selected.
— end note]
(4.2)
Otherwise, if the types of E1 and E2 each model indirectly_readable, and if the reference types of E1 and E2 model swappable_with ([concept.swappable]), then ranges::swap(*E1, *E2).
(4.3)
Otherwise, if the types T1 and T2 of E1 and E2 model indirectly_movable_storable<T1, T2> and indirectly_movable_storable<T2, T1>, then (void)(*E1 = iter-exchange-move(E2, E1)), except that E1 is evaluated only once.
(4.4)
Otherwise, ranges::iter_swap(E1, E2) is ill-formed.

[Note 2:
This case can result in substitution failure when ranges::iter_swap(E1, E2) appears in the immediate context of a template instantiation.
— end note]