Add unit tests for presorted inputs to sorting (#48638)

* Add unit tests for presorted inputs to sorting

base/sort.jl

@@ -785,7 +785,7 @@ function _sort!(v::AbstractVector, a::CheckSorted, o::Ordering, kw)
 
     # For most large arrays, a reverse-sorted check is essentially free (overhead < 1%)
     if hi-lo >= 500 && _issorted(v, lo, hi, ReverseOrdering(o))
-        # If reversing is valid, do so. This does violates stability.
+        # If reversing is valid, do so. This violates stability.
         reverse!(v, lo, hi)
         return scratch
     end

test/sorting.jl

@@ -950,6 +950,17 @@ end
     test_allocs()
 end
 
+@testset "Presorted and reverse-presorted" begin
+    for len in [7, 92, 412, 780]
+        x = sort(randn(len))
+        for _ in 1:2
+            @test issorted(sort(x))
+            @test issorted(sort(x), by=x -> x+7)
+            reverse!(x)
+        end
+    end
+end
+
 # This testset is at the end of the file because it is slow.
 @testset "searchsorted" begin
     numTypes = [ Int8,  Int16,  Int32,  Int64,  Int128,