, if either
has class or enumeration type and
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
the program is ill-formed, no diagnostic required.