How do I specify a custom hash function explicitly for unordered_set by passing a named function?

Based on the accepted answer to this question, one can use a specialization to std to provide a hash function for a user defined type.

#include <unordered_set>
#include <stdint.h>


struct FooBar {
    int i; 
};
namespace std {
    template <> struct hash<FooBar>
    {
        size_t operator()(const FooBar & x) const
        {
            return x.i;
        }
    };
}

int main(){
    std::unordered_set<FooBar> foo(0);
}

However, the documentation seems to imply that a custom hash function can also be passed explicitly into the constructor, and I would like to use a named function for this hash function.

However, my current attempt suffers from compile errors.

#include <unordered_set>
#include <stdint.h>

struct FooBar {
    int i; 
};

const size_t hashFooBar(const FooBar& foo) {
    return foo.i;
}

int main(){
    std::unordered_set<FooBar> foo(0, hashFooBar);
}

What is the correct template magic and method signature to make this work?