From 75ba566bb658e7979a4a19dc2f1c124ebb9a898e Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Sat, 5 Aug 2017 13:29:28 -0400
Subject: [PATCH] deprecate `union()`

---
 NEWS.md            | 3 +++
 base/deprecated.jl | 2 ++
 base/set.jl        | 1 -
 test/sets.jl       | 1 -
 4 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 104a4b49500ae..29cf728efc970 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -262,6 +262,9 @@ Deprecated or removed
     full path if you need access to executables or libraries in the `JULIA_HOME` directory, e.g.
     `joinpath(JULIA_HOME, "7z.exe")` for `7z.exe` ([#21540]).
 
+  * Calling `union` with no arguments is deprecated; construct an empty set with an appropriate
+    element type using `Set{T}()` instead ([#23144]).
+
 Julia v0.6.0 Release Notes
 ==========================
 
diff --git a/base/deprecated.jl b/base/deprecated.jl
index bd90c1497825d..5e63afcff4a1e 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -1334,6 +1334,8 @@ end
 
 @deprecate issubtype (<:)
 
+@deprecate union() Set()
+
 # 12807
 start(::Union{Process, ProcessChain}) = 1
 done(::Union{Process, ProcessChain}, i::Int) = (i == 3)
diff --git a/base/set.jl b/base/set.jl
index c0d18af570d61..7d79287e4abe2 100644
--- a/base/set.jl
+++ b/base/set.jl
@@ -65,7 +65,6 @@ done(s::Set, state) = done(s.dict, state)
 # NOTE: manually optimized to take advantage of Dict representation
 next(s::Set, i)     = (s.dict.keys[i], skip_deleted(s.dict, i+1))
 
-union() = Set()
 union(s::Set) = copy(s)
 function union(s::Set, sets::Set...)
     u = Set{join_eltype(s, sets...)}()
diff --git a/test/sets.jl b/test/sets.jl
index 8808036301acb..6e62708f64ced 100644
--- a/test/sets.jl
+++ b/test/sets.jl
@@ -138,7 +138,6 @@ for data_in in ((7,8,4,5),
 end
 
 # union
-@test isequal(union(),Set())
 @test isequal(union(Set([1])),Set([1]))
 s = ∪(Set([1,2]), Set([3,4]))
 @test isequal(s, Set([1,2,3,4]))