Fix sort by always using QuickSort

`MergeSort` is used by default, and it fails since it attempts to compare categorical values
from different pools. Always use `QuickSort`, which gives the same result anyway since
categorical values are always strictly ordered, and is as fast as sorting an integer array.

src/array.jl

@@ -797,4 +797,16 @@ Base.Broadcast.broadcasted(::typeof(ismissing), A::CategoricalArray{T}) where {T
 
 Base.Broadcast.broadcasted(::typeof(!ismissing), A::CategoricalArray{T}) where {T} =
     T >: Missing ? Base.Broadcast.broadcasted(>, A.refs, 0) :
-                   Base.Broadcast.broadcasted(_ -> true, A.refs)
+                   Base.Broadcast.broadcasted(_ -> true, A.refs)
+
+function Base.sort!(v::CategoricalVector;
+                    alg::Base.Algorithm=QuickSort,
+                    lt=isless,
+                    by=identity,
+                    rev::Bool=false,
+                    order::Base.Ordering=Base.Forward)
+    # MergeSort errors when trying to compare values from different pools
+    # Stability does not make a difference anyway since categorical values
+    # are always strictly ordered
+    invoke(sort!, Tuple{AbstractVector}, v, alg=QuickSort, lt=lt, by=by, rev=rev, order=order)
+end

test/13_arraycommon.jl

@@ -689,6 +689,23 @@ end
         @test sort(x) == ["Young", "Young", "Middle", "Old"]
         @test sort!(x) === x
         @test x == ["Young", "Young", "Middle", "Old"]
+        @test levels(x) == ["Young", "Middle", "Old"]
+
+        # Default sort method uses MergeSort (which fails) only beyond
+        # a certain number of elements
+        if T === Missing
+            y = rand([1:10; missing], 1000)
+        else
+            y = rand(1:10, 1000)
+        end
+        cy = categorical(y)
+        levels!(cy, collect(10:-1:1))
+        res = [sort(collect(skipmissing(y)), rev=true);
+               fill(missing, count(ismissing, y))]
+        @test sort(cy) ≅ res
+        @test sort!(cy) === cy
+        @test cy ≅ res
+        @test levels(cy) == 10:-1:1
     end
 end