Recursive map works, but recursive unordered_map doesn't

Question

I was wondering, why does this code compile just fine with g++,

struct FileSystemMap {
    std::map<std::string, FileSystemMap> directories;
    std::map<std::string, std::string> files;
};

while this other doesn't?

struct FileSystemMap {
    std::unordered_map<std::string, FileSystemMap> directories;
    std::unordered_map<std::string, std::string> files;
};