Union{}[] cannot be displayed

[BioTurboNick]

The following code leads to a display error (see end):

using NearestNeighbors
map([]) do f
	knn(BallTree(), [])
end # Union{}[]

The underlying issue I believe is that since the interior is invalid, type inference is failing, but it's odd that Pluto can't display the result. (Unless this is a Julia bug and this type should never be observed?)

Compared to:

map([]) do f
end # Nothing[]

map([]) do f
    knn(BallTree([1.0; 2.0]), [1.0; 2.0], 1)
end # Any[]

Failed to show value:

MethodError: show(::IOContext{IOBuffer}, ::MIME{Symbol("image/svg+xml")}, ::Vector{Union{}}) is ambiguous. Candidates:

show(io::IO, mime::MIME{Symbol("image/svg+xml")}, cs::AbstractVector{T}; max_swatches) where T<:ColorTypes.TransparentColor in Colors at /home/ec2-user/.julia/packages/Colors/yDxFN/src/display.jl:41

show(io::IO, mime::MIME{Symbol("image/svg+xml")}, cs::AbstractVector{T}; max_swatches) where T<:ColorTypes.Color in Colors at /home/ec2-user/.julia/packages/Colors/yDxFN/src/display.jl:35

Possible fix, define

show(::IO, ::MIME{Symbol("image/svg+xml")}, ::AbstractVector{Union{}})

show_richest(::IOContext{IOBuffer}, ::Any)@PlutoRunner.jl:1143
show_richest_withreturned@PlutoRunner.jl:1068[inlined]
format_output_default(::Any, ::Any)@PlutoRunner.jl:968
var"#format_output#51"(::IOContext{Base.DevNull}, ::typeof(Main.PlutoRunner.format_output), ::Any)@PlutoRunner.jl:985
formatted_result_of(::Base.UUID, ::Base.UUID, ::Bool, ::Vector{String}, ::Nothing, ::Module)@PlutoRunner.jl:878
top-level scope@WorkspaceManager.jl:477

[fonsp]

It looks like this is a Colors.jl issue: JuliaGraphics/Colors.jl#526