template <class T> // comment
auto foo(T x) { ... };

namespace impl {
    template<typename T>
    struct is_streamable<T, std::void_t<decltype(std::declval<std::wostream &>() << std::declval<T>())>> : std::true_type { };
}

// Disable overload for already valid operands.
template<class T, class = std::enable_if_t<!impl::is_streamable_v<const T &> && std::is_convertible_v<const T &, std::wstring_view>>>
std::wostream &operator <<(std::wostream &stream, const T &thing)
{
    return stream << static_cast<std::wstring_view>(thing);
}

enum struct DataHolder { };
enum class DataThingy { };
enum class Boolean : char {
    True, False, FileNotFound
};

union Soy
{
};