Define OrderStyle for Union{} (#36810)

Removes ambiguity: ``` julia> Base.OrderStyle(Union{}) ERROR: MethodError: Base.OrderStyle(::Type{Union{}}) is ambiguous. Candidates: ``` This error is relevant as with current `unique!` definition that relies on `OrderStyle` one can have a problem in corner cases. E.g.: ``` julia> [i for i in ["1"] if i isa Int] 0-element Array{Union{},1} ``` Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>
JuliaLang · Aug 13, 2020 · 03e1a89 · 03e1a89 · nanosoldier · Aug 13, 2020
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diff --git a/base/traits.jl b/base/traits.jl
@@ -11,6 +11,7 @@ OrderStyle(::Type{<:Real}) = Ordered()
 OrderStyle(::Type{<:AbstractString}) = Ordered()
 OrderStyle(::Type{Symbol}) = Ordered()
 OrderStyle(::Type{<:Any}) = Unordered()
+OrderStyle(::Type{Union{}}) = Ordered()
 
 # trait for objects that support arithmetic
 abstract type ArithmeticStyle end

diff --git a/test/sets.jl b/test/sets.jl
@@ -447,6 +447,9 @@ end
     @test @inferred(unique!(iseven, [2, 3, 5, 7, 9])) == [2, 3]
     @test @inferred(unique!(x -> x % 2 == 0 ? :even : :odd, [1, 2, 3, 4, 2, 2, 1])) == [1, 2]
     @test @inferred(unique!(x -> x % 2 == 0 ? :even : "odd", [1, 2, 3, 4, 2, 2, 1]; seen=Set{Union{Symbol,String}}())) == [1, 2]
+
+    @test isempty(unique!(Union{}[]))
+    @test eltype(unique!([i for i in ["1"] if i isa Int])) <: Union{}
 end
 
 @testset "allunique" begin