From 41058485fce3a8b2770dbc68379c49b90ccdc9c7 Mon Sep 17 00:00:00 2001
From: John Lapeyre <jlapeyre@users.noreply.github.com>
Date: Thu, 4 Apr 2019 17:05:07 +0200
Subject: [PATCH] Add methods to `reduce`, specialized for `merge` on vector of
 Dict (#26440)

---
 base/dict.jl |  6 ++++++
 test/dict.jl | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/base/dict.jl b/base/dict.jl
index 9cd3fe5009de1..ab6e636397838 100644
--- a/base/dict.jl
+++ b/base/dict.jl
@@ -691,6 +691,12 @@ end
 
 filter!(f, d::Dict) = filter_in_one_pass!(f, d)
 
+function reduce(::typeof(merge), items::Vector{<:Dict})
+    K = mapreduce(keytype, promote_type, items)
+    V = mapreduce(valtype, promote_type, items)
+    return reduce(merge!, items; init=Dict{K,V}())
+end
+
 function map!(f, iter::ValueIterator{<:Dict})
     dict = iter.dict
     vals = dict.vals
diff --git a/test/dict.jl b/test/dict.jl
index fd691ce8a62b8..904cdb9526c57 100644
--- a/test/dict.jl
+++ b/test/dict.jl
@@ -931,6 +931,23 @@ end
     @test d1 == Dict("A" => 1, "B" => 15, "C" => 28)
 end
 
+@testset "Dict reduce merge" begin
+    function check_merge(i::Vector{<:Dict}, o)
+        r1 = reduce(merge, i)
+        r2 = merge(i...)
+        t = typeof(o)
+        @test r1 == o
+        @test r2 == o
+        @test typeof(r1) == t
+        @test typeof(r2) == t
+    end
+    check_merge([Dict(1=>2), Dict(1.0=>2.0)], Dict(1.0=>2.0))
+    check_merge([Dict(1=>2), Dict(2=>Complex(1.0, 1.0))],
+      Dict(2=>Complex(1.0, 1.0), 1=>Complex(2.0, 0.0)))
+    check_merge([Dict(1=>2), Dict(3=>4)], Dict(3=>4, 1=>2))
+    check_merge([Dict(3=>4), Dict(:a=>5)], Dict(:a => 5, 3 => 4))
+end
+
 @testset "misc error/io" begin
     d = Dict('a'=>1, 'b'=>1, 'c'=> 3)
     @test_throws ErrorException 'a' in d