setdiff preserves eltype (#59439)

Author: adienes

base/abstractset.jl

@@ -201,7 +201,7 @@ intersect!(s::AbstractSet, itr) =
     setdiff(s, itrs...)
 
 Construct the set of elements in `s` but not in any of the iterables in `itrs`.
-Maintain order with arrays.
+Maintain order with arrays. The result will have the same element type as `s`.
 
 See also [`setdiff!`](@ref), [`union`](@ref) and [`intersect`](@ref).
 
@@ -211,6 +211,10 @@ julia> setdiff([1,2,3], [3,4,5])
 2-element Vector{Int64}:
  1
  2
+
+julia> setdiff([1,2,3], [1.0, 2.0])
+1-element Vector{Int64}:
+ 3
 ```
 """
 setdiff(s::AbstractSet, itrs...) = setdiff!(copymutable(s), itrs...)

base/array.jl

@@ -3133,14 +3133,17 @@ setdiff!(  v::AbstractVector, itrs...) = _shrink!(setdiff!, v, itrs)
 
 vectorfilter(T::Type, f, v) = T[x for x in v if f(x)]
 
-function _shrink(shrinker!::F, itr, itrs) where F
+function intersect(itr, itrs...)
     T = promote_eltype(itr, itrs...)
-    keep = shrinker!(Set{T}(itr), itrs...)
+    keep = intersect!(Set{T}(itr), itrs...)
     vectorfilter(T, _shrink_filter!(keep), itr)
 end
 
-intersect(itr, itrs...) = _shrink(intersect!, itr, itrs)
-setdiff(  itr, itrs...) = _shrink(setdiff!, itr, itrs)
+function setdiff(itr, itrs...)
+    T = eltype(itr)
+    keep = setdiff!(Set{T}(itr), itrs...)
+    vectorfilter(T, _shrink_filter!(keep), itr)
+end
 
 function intersect(v::AbstractVector, r::AbstractRange)
     T = promote_eltype(v, r)

test/arrayops.jl

@@ -1246,6 +1246,18 @@ end
         @test setdiff((1, 2), (3, 2)) == [1]
         @test symdiff((1, 2), (3, 2)) == [1, 3]
     end
+
+    @testset "setdiff preserves element type of first argument" begin
+        @test setdiff([1, 2, 3], [1.0, 2.0]) isa Vector{Int}
+        @test setdiff([1.0, 2.0, 3.0], [1, 2]) isa Vector{Float64}
+        @test setdiff(['a', 'b', 'c'], [98]) isa Vector{Char}
+        @test setdiff([1, 2], [1.0, 2.0]) isa Vector{Int}
+    end
+    @testset "intersect promotes element types of arguments" begin
+        @test intersect([1, 2, 3], [1.0, 2.0]) isa Vector{Float64}
+        @test intersect([1.0, 2.0, 3.0], [1, 2]) isa Vector{Float64}
+        @test intersect(['a', 'b', 'c'], Int[]) isa Vector{Any}
+    end
 end
 
 @testset "mapslices" begin