What is the return value of the C$++20$ function fn defined below?

Binclude

# include $$optional$>$

#include $$vector$>$

auto fn $($const std:: vector$$int$>$ &vec$)->$ std::optional $($

auto iter $=$ vec. cbegin$()$ ;

while $($iter $|=$ vec.cend$))\{$

if $($std:: count$\_$if $($iter$,$ vec.cend$(),[$iter$]($auto lhs$)($ return lhs $$iter; $))=0)($

$+$titer$$

continue;

$\}$

return $*$iter;

$\}$

return std: :nullopt;

$\}$

an optional containing the largest value in the input vector, or an empty optional if the input vector is empty

an optional containing the smallest value in the input vector, or an empty optional if the input vector is empty

an optional containing the first duplicated value, or an empty optional if all elements in the input vector are unique

an optional containing the most frequently occurring value in the input vector, and never an empty optional

this function will not compile due to a syntax error