Array of unions of subtypes

New to Julia
1

What is the difference between Array{Union{Nothing, <:Real}} and Array{<:Union{Nothing, Real}}?
I expect Vector{Int64} to be a subtype of the array of the union of nothing and subtypes of real, but only the latter gives true. What is the meaning of <:Union{Nothing, Real}?

2

Array{<:Union{Nothing, Real}} is a syntax sugar for Array{T} where {T<:Union{Nothing, Real}}.

It’s called UnionAll which means Union{Array{Nothing}, Array{Real}}. See the talk linked below for more details.

3

Union{Array{Nothing}, Array{Real}} doesn’t seem to include Vector{Union{Nothing, Int64}} … or does it?
So now my understanding is that Union{Nothing, Real} isn’t simply a union of type Nothing and abstract type Real, but also includes all concrete subtypes in the set. I suppose Union{Nothing, <:Real} doesn’t mean the way I expect it to be, as in the union of type Nothing and all of the concrete subtypes of Real.

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4 
julia> x = Array{T} where T<:Union{Nothing,S} where S <: Real
Array{T} where {S<:Real, T<:Union{Nothing, S}}

julia> y = Array{Union{Nothing, <:Real}} 
Array{Union{Nothing, Real}}

julia> x == y
false

julia> y <: x
true

julia> x <: y
false
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