Base.show(io, ::MIME"image/svg+xml", ::Vector{Union{}}) is ambiguous

[Roger-luo]

In Colors, this is overloaded for the Color type, but because Union{} <: Color, this will be ambiguous, e.g

using Colors
st = Union{}[]
Base.show(stdout, MIME"image/svg+xml"(), st)

gives

julia> Base.show(stdout, MIME"image/svg+xml"(), st)
ERROR: MethodError: show(::Base.TTY, ::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<:TransparentColor in Colors at /Users/roger/.julia/packages/Colors/yDxFN/src/display.jl:41
  show(io::IO, mime::MIME{Symbol("image/svg+xml")}, cs::AbstractVector{T}; max_swatches) where T<:Color in Colors at /Users/roger/.julia/packages/Colors/yDxFN/src/display.jl:35
Possible fix, define
  show(::IO, ::MIME{Symbol("image/svg+xml")}, ::AbstractVector{Union{}})
Stacktrace:
 [1] top-level scope
   @ REPL[11]:1

because show(::IO, ::MIME{Symbol("image/svg+xml")}, ::AbstractVector{Union{}}) only relats to types in Base, so I think it should be defined here, but the questions how should we define it?

[thofma]

One can construct infinitely many ambiguities, which, following this logic, should be resolved in Base:

julia> struct MyType1 end

julia> struct MyType2 end

julia> Base.:(+)(x::Vector{T}, y::Vector{T}) where {T <: MyType1} = x .+ y

julia> Base.:(+)(x::Vector{T}, y::Vector{T}) where {T <: MyType2} = x .+ y

julia> Union{}[] + Union{}[]
ERROR: MethodError: +(::Vector{Union{}}, ::Vector{Union{}}) is ambiguous. Candidates:
  +(x::Vector{T}, y::Vector{T}) where T<:MyType1 in Main at REPL[3]:1
  +(x::Vector{T}, y::Vector{T}) where T<:MyType2 in Main at REPL[4]:1
Possible fix, define
  +(::Vector{Union{}}, ::Vector{Union{}})
Stacktrace:
 [1] top-level scope
   @ REPL[5]:1

I don't think adding random methods to Base is a solution that scales.

[simeonschaub]

I have to agree. Defining methods for AbstractVector{<:T} is considered type piracy, so this needs to be fixed at a package level. See also #33780