Question about internal representation of Union{Missing, Float64}

[Tamas_Papp]

cf

sizeof(::Vector{Union{missing,T}}), and hypothetical size

Two (related) questions: julia> sizeof(vcat(ones(10), fill(missing, 10))) 160 but I expected 20*9 because of the type tag, what am I missing? v"1.1.0-DEV.298". If I have a type T which is a Union, can I calculate the size of a Vector{T} of n elements without actually creating one?