Is it possible to have user-defined functions with vector input in JuMP 0.18.6? Or is there a way to do similar things? I would like to use the NLopt solver (https://github.com/JuliaOpt/NLopt.jl) and it seems that this solver is not supported after JuMP 0.18.

For example, is it possible to do something like

```
function f(x)
return dot(x,x)
end
function g(x)
return dot(x,x)
end
Model = (model, solver=NLoptSolver(algorithm=:LD_MMA))
@variable(model, x[1:n])
@objective(model, Min, f(x))
@constraint(model, g(x)<=0)
solve(model)
```

For n large, it will be very time-consuming to list x[1],…,x[n] as scalar one by one.

It seems that we can do the vector input by Julia’s splatting syntax in JuMP0.21.3, as shown below:

I tried this in JuMP 0.18.6 but got an error.

Another question is, if the solver I use doesn’t require derivative, do I need to set `autodiff=false`

?