no promotion exists for ForwardDiff.Dual{N,T<:Real} and Float64

Hi all !

I would like to concatenate a vector of Dual with a Real as the following

T = 1
p = Array{Dual}(N)
k = 2
ps = vcat(p[1:k], T)
# z = rand(Categorical(ps))

Yet I get the error no promotion exists for ForwardDiff.Dual{N,T<:Real} and Float64.
Therefore I tried to extends the vcat function:

function Base.vcat{T<:Real}(a::Array{Dual{T}}, x::T)
  N = length(a)
  new_a = Array{Dual}(N+1)
  new_a[1:N] = a
  new_a[N+1] = Dual(x)
  new_a
end

Unfortunatly this does not work.
How shall I proceed ?

Thanks for your help,
Emile

Start by reproducing the Julia snippet in the issue and trace how ForwardDiff.Dual values are handled during vcat and numeric promotion. Determine the intended behavior for concatenating Dual and Real values, then verify that the example no longer raises the reported promotion error.