[julia-users] Avoid type Unions in fields?

[Gnimuc Key]

>
> Avoid type Unions in fields ¶
>
> <http://docs.julialang.org/en/latest/manual/style-guide/#avoid-strange-type-unions>
> type MyType
> ...
> x::Union{Void,T}
> end

 
I'm wondering whether it's a good style or not to write something like this:

type MyType
    ...
    x::Union{Int64,Float64}end

what's the side effects of using Union like this?