From 6029d10d755a36b9305e2f86557008beb8be651f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Denis=20Baru=C4=8Di=C4=87?= <barucden@fel.cvut.cz>
Date: Mon, 27 Sep 2021 16:32:24 +0200
Subject: [PATCH] Base: `intersect` keep the container type

A bug was introduced for 3 arguments version of `intersect` in #41769.
The container type always changed to `Set`:

```
julia> intersect(BitSet([1,2]), [1,2], [2])
Set{Int64} with 1 element:
  2
```

This is an attempt to return to the original behavior:

```
julia> intersect(BitSet([1,2]), [1,2], [2])
BitSet with 1 element:
  2
```
---
 base/abstractset.jl | 8 +++++++-
 test/sets.jl        | 6 ++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/base/abstractset.jl b/base/abstractset.jl
index c4ec114c1a645..fb17f250b4b72 100644
--- a/base/abstractset.jl
+++ b/base/abstractset.jl
@@ -129,7 +129,13 @@ Set{Int64} with 1 element:
 """
 function intersect(s::AbstractSet, itr, itrs...)
     T = promote_eltype(s, itr, itrs...)
-    return intersect!(Set{T}(s), itr, itrs...)
+    if T == promote_eltype(s, itr)
+        out = intersect(s, itr)
+    else
+        out = union!(emptymutable(s, T), s)
+        intersect!(out, itr)
+    end
+    return intersect!(out, itrs...)
 end
 intersect(s) = union(s)
 intersect(s::AbstractSet, itr) = mapfilter(in(s), push!, itr, emptymutable(s, promote_eltype(s, itr)))
diff --git a/test/sets.jl b/test/sets.jl
index b49507876265d..5e5857e599a5b 100644
--- a/test/sets.jl
+++ b/test/sets.jl
@@ -254,6 +254,8 @@ end
     end
     @test intersect(Set([1]), BitSet()) isa Set{Int}
     @test intersect(BitSet([1]), Set()) isa Set{Any}
+    @test intersect(BitSet([1]), Set([1])) isa BitSet
+    @test intersect(BitSet([1]), Set([1]), Set([1])) isa BitSet
     @test intersect([1], BitSet()) isa Vector{Int}
     # intersect must uniquify
     @test intersect([1, 2, 1]) == intersect!([1, 2, 1]) == [1, 2]
@@ -276,6 +278,10 @@ end
             @test eltype(intersect(a, a, b)) == Float64
         end
     end
+
+    # 3-argument version is correctly covered
+    @test intersect(Set([1,2]), Set([2]), Set([1,2,3])) == Set([2])
+    @test intersect(Set([1,2]), Set([2]), Set([1.,2,3])) == Set([2.])
 end
 
 @testset "setdiff" begin