From eab80001e74307baedae57e2160f6ad1dd492b40 Mon Sep 17 00:00:00 2001
From: nseaSeb <nseaprotector@gmail.com>
Date: Fri, 10 Jul 2026 10:03:50 +0200
Subject: [PATCH] Fix Range.disjoint?/2 for single-element ranges with a
 negative step

`member?/2`, `size/1` and `to_list/1` all treat a single-element range
such as `1..1//-2` as the set `[1]`, and `Range.split/2` produces such
ranges from library code (e.g. `Range.split(5..1//-2, 2)` yields
`1..1//-2`). Yet `disjoint?/2` reported two of them as sharing no
element:

    Range.disjoint?(1..1//-2, 1..1//-2)  #=> true (should be false)

`normalize/3` only reversed ranges when `first > last`, leaving a
negative step in place for single-element ranges (`first == last`). The
arithmetic-progression intersection in `disjoint?/2` assumes an
increasing progression, so the leftover negative step produced a wrong
result. The `step == -1` case was masked by the `abs(step) == 1`
shortcut, so only `step <= -2` was affected.

Assisted-by: Claude Code:claude-opus-4-8
---
 lib/elixir/lib/range.ex               | 4 ++++
 lib/elixir/test/elixir/range_test.exs | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/lib/elixir/lib/range.ex b/lib/elixir/lib/range.ex
index 1ed62a0c59d..58886331864 100644
--- a/lib/elixir/lib/range.ex
+++ b/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
diff --git a/lib/elixir/test/elixir/range_test.exs b/lib/elixir/test/elixir/range_test.exs
index 16009818b55..508d2d29178 100644
--- a/lib/elixir/test/elixir/range_test.exs
+++ b/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