Redefining the same type does not work if fields have UnionAll types

[simonster]

I'm not 100% sure this is "officially supported" but I've been led to believe it's supposed to work:

julia> struct X
           x::Array{T} where T<:Real
       end

julia> struct X
           x::Array{T} where T<:Real
       end
ERROR: invalid redefinition of constant X

Tested with 3777aab