The name ranges::iter_swap denotes
a customization point object ([customization.point.object])
that exchanges the values ([concept.swappable]) denoted by its
arguments.

```
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 some subexpressions
E1 and E2 is expression-equivalent to:

- (void)iter_swap(E1, E2), if that expression is valid,
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 with no diagnostic required. - 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).
- 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.
- Otherwise, ranges::iter_swap(E1, E2) is ill-formed. [ Note: This case can result in substitution failure when ranges::iter_swap(E1, E2) appears in the immediate context of a template instantiation. â€”]
*end note*