Fix Range.disjoint?/2 for single-element ranges with a negative step

lib/elixir/lib/range.ex

@@ -517,6 +517,10 @@ defmodule Range do
   defp normalize(first, last, step) when first > last,
     do: {first - abs(div(first - last, step) * step), first, -step}
 
+  # A single-element range holds the same element regardless of the step, so
+  # make the step positive to keep the progression in disjoint?/2 increasing.
+  defp normalize(first, last, step) when first == last, do: {first, last, abs(step)}
+
   defp normalize(first, last, step), do: {first, last, step}
 
   @doc false

lib/elixir/test/elixir/range_test.exs

@@ -103,6 +103,11 @@ defmodule RangeTest do
       assert_overlap(-7..-5, -5..-1)
 
       assert Range.disjoint?(1..1, 1..1) == false
+
+      # Single-element ranges still contain their element regardless of the step
+      assert Range.disjoint?(1..1//-2, 1..1//-2) == false
+      assert Range.disjoint?(3..3//-3, 1..5) == false
+      assert Range.disjoint?(1..5, 3..3//-3) == false
     end
   end