What’s the function signature need to look like for the first argument of stan::math::hessian?

I have a signature like:

```
template <typename T_x>
inline const return_type_t<T_x>
f2(const std::vector<T_x> &x)
```

and the compiler doesn’t like the first argument, gives: `<unresolved overloaded function type>`

.

How should I define `const F &`

, the first argument for stan::math::hessian?

Thanks!