Preserve the input element type in unique (#23208)

Previously the element type of the output was the smallest type that
would fit the union of the input's individual element types. Now the
output has an identical element type to the input. Fixes #22696.

NEWS.md

@@ -168,6 +168,10 @@ This section lists changes that do not have deprecation warnings.
     way as `^(A::Integer, p::Integer)`. This means, for instance, that `[1 1; 0 1]^big(1)`
     will return a `Matrix{BigInt}` instead of a `Matrix{Int}` ([#23366]).
 
+  * The element type of the input is now preserved in `unique`. Previously the element type
+    of the output was shrunk to fit the union of the type of each element in the input.
+    ([#22696])
+
 Library improvements
 --------------------
 
@@ -1204,6 +1208,7 @@ Command-line option changes
 [#22588]: https://github.com/JuliaLang/julia/issues/22588
 [#22605]: https://github.com/JuliaLang/julia/issues/22605
 [#22666]: https://github.com/JuliaLang/julia/issues/22666
+[#22696]: https://github.com/JuliaLang/julia/issues/22696
 [#22703]: https://github.com/JuliaLang/julia/issues/22703
 [#22712]: https://github.com/JuliaLang/julia/issues/22712
 [#22718]: https://github.com/JuliaLang/julia/issues/22718

base/set.jl

@@ -240,7 +240,8 @@ const ⊆ = issubset
 
 Return an array containing only the unique elements of collection `itr`,
 as determined by [`isequal`](@ref), in the order that the first of each
-set of equivalent elements originally appears.
+set of equivalent elements originally appears. The element type of the
+input is preserved.
 
 # Examples
 ```jldoctest
@@ -249,6 +250,11 @@ julia> unique([1, 2, 6, 2])
  1
  2
  6
+
+julia> unique(Real[1, 1.0, 2])
+2-element Array{Real,1}:
+ 1
+ 2
 ```
 """
 function unique(itr)
@@ -260,7 +266,7 @@ function unique(itr)
         return out
     end
     x, i = next(itr, i)
-    if !isleaftype(T)
+    if !isleaftype(T) && iteratoreltype(itr) == EltypeUnknown()
         S = typeof(x)
         return _unique_from(itr, S[x], Set{S}((x,)), i)
     end

test/core.jl

@@ -5382,7 +5382,8 @@ for U in unboxedunions
             # deleteat!
             F = Base.uniontypes(U)[2]
             A = U[rand(F(1):F(len)) for i = 1:len]
-            deleteat!(A, map(Int, sort!(unique(A[1:4]))))
+            # The 2-arg `unique` method works around #22688
+            deleteat!(A, map(Int, sort!(unique(identity, A[1:4]))))
             A = U[initvalue2(F2) for i = 1:len]
             deleteat!(A, 1:2)
             @test length(A) == len - 2

test/sets.jl

@@ -233,6 +233,8 @@ u = unique([1,1,2])
 # issue 20105
 @test @inferred(unique(x for x in 1:1)) == [1]
 @test unique(x for x in Any[1,1.0])::Vector{Real} == [1]
+@test unique(x for x in Real[1,1.0])::Vector{Real} == [1]
+@test unique(Integer[1,1,2])::Vector{Integer} == [1,2]
 
 # unique!
 @testset "unique!" begin