From 03e1a89ff1f62f05cf3937f5da44f3d46956138a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bogumi=C5=82=20Kami=C5=84ski?= <bkamins@sgh.waw.pl>
Date: Thu, 13 Aug 2020 16:37:29 +0200
Subject: [PATCH] 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>
---
 base/traits.jl | 1 +
 test/sets.jl   | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/base/traits.jl b/base/traits.jl
index b7239230686c8..53ae14b12c61e 100644
--- 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
index 2f08ecb210cbb..b090a3d7097fa 100644
--- 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