Consider: this thread
And this example:

struct NonCopyable {
    NonCopyable() = default;
    ~NonCopyable() {}
private:
    std::unique_ptr<int> value;
};
template <typename T>
struct Wrapper {
    Wrapper() = default;
    Wrapper(const Wrapper&) = default;
    Wrapper(Wrapper&&) = default;
    T* v = nullptr;
};

Wrapper<NonCopyable> globalFunction() { return {}; }

and registering globalFunction with:

lua["globalFunction"] = &globalFunction;

Compiling with clang gives: call to implicitly-deleted copy constructor of 'Wrapper<NonCopyable>', so sol is relying on std::is_copy_constructible which should be generally enough (enough for non templated types).
But for types like the example above, we can use a specialized trait to modify the behavior (quite smooth if we are using a custom code generator).

sol documentation should shed some light on type traits, especially the comparison traits because for wrapped types (not pushed as custom types), we can't rely on std <type_traits> alone.