public Map<Integer, Collection<String>> myFunction(final List<Collection<String>> strs) {
    return strs.stream().collect(Collectors.toMap(List::indexOf, v -> v)); // Error here
}

return strs.stream().collect(Collectors.toMap(coll -> strs.indexOf(coll), v -> v));

return strs.stream().collect(Collectors.toMap(strs::indexOf, v -> v));