Actually, the corner case for [over.ics.list] p5 is that

#include <initializer_list>
struct A{
    operator short(){
        return 0;
    }
};
struct B{
    operator bool(){
        return 0;
    }
};
void fun(std::initializer_list<int>){}  //#1
void fun(std::initializer_list<bool>){}  //#2
int main(){
    fun({A{},B{}});
}

The worst conversion sequence selected in the implicit conversion sequence set of #1 will directly impact the result of comparing it with the obvious worst conversion sequence (A::short) in the set of #2. Regardless of A::short or B::bool, they are all the implicit conversion sequence that comprises a user-defined conversion followed by an integral promotion. Which is not better or worse than the other. If we consider A::short is the worst conversion sequence in the set of #1, then #1 is selected as per [over.match.best#over.ics.rank-3.3]. Otherwise, the invocation will be ambiguous.