From b70b46f77836b5e3f83300d9ba046b0ae973dca3 Mon Sep 17 00:00:00 2001
From: pnezis <pnezis@gmail.com>
Date: Mon, 13 Jul 2026 00:56:02 +0300
Subject: [PATCH] Fix `Enum.min/2,3` and `Enum.max/2,3` with a custom sorter on
 ranges

The range clause of `aggregate/3` only compared the two endpoints of the
range, which is correct solely for sorters consistent with the natural
integer order. Any other comparator returned a different result than
the identical call on the equivalent list or stream:

    sorter = fn a, b -> rem(a, 5) >= rem(b, 5) end
    Enum.max(1..10, sorter)               #=> 1, expected 4
    Enum.max(Enum.to_list(1..10), sorter) #=> 4

Once the range is known to be non-empty, take the endpoint shortcut
only when the sorter is one of the defaults (`&>=/2` or `&<=/2`, both
order-consistent); any other sorter traverses the elements through
`reduce_range/5`, seeded with the first element.
---
 lib/elixir/lib/enum.ex               | 23 ++++++++++++++++++-----
 lib/elixir/test/elixir/enum_test.exs | 16 ++++++++++++++++
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/lib/elixir/lib/enum.ex b/lib/elixir/lib/enum.ex
index 3ca9a3f3d4b..f5af8e51346 100644
--- a/lib/elixir/lib/enum.ex
+++ b/lib/elixir/lib/enum.ex
@@ -4309,11 +4309,24 @@ defmodule Enum do
         empty.()
 
       _ ->
-        last = last - rem(last - first, step)
-
-        case fun.(first, last) do
-          true -> first
-          false -> last
+        # The endpoint shortcut is only valid for sorters consistent with
+        # the natural integer order of the range elements, which is known
+        # to hold for the default sorters; any other sorter traverses the
+        # elements, seeded with the first one since the range is not empty
+        if fun == (&<=/2) or fun == (&>=/2) do
+          last = last - rem(last - first, step)
+
+          case fun.(first, last) do
+            true -> first
+            false -> last
+          end
+        else
+          reduce_range(first + step, last, step, first, fn element, acc ->
+            case fun.(acc, element) do
+              true -> acc
+              false -> element
+            end
+          end)
         end
     end
   end
diff --git a/lib/elixir/test/elixir/enum_test.exs b/lib/elixir/test/elixir/enum_test.exs
index 1be9e4a2eee..b001776757f 100644
--- a/lib/elixir/test/elixir/enum_test.exs
+++ b/lib/elixir/test/elixir/enum_test.exs
@@ -2096,6 +2096,14 @@ defmodule EnumTest.Range do
     assert Enum.max(1..2, fn -> 0 end) === 2
   end
 
+  test "max/2 with custom sorter" do
+    sorter = fn a, b -> rem(a, 5) >= rem(b, 5) end
+    assert Enum.max(1..10, sorter) == 4
+    assert Enum.max(1..10//2, sorter) == 9
+    assert Enum.max(10..1//-1, sorter) == 9
+    assert Enum.max(1..0//1, sorter, fn -> :empty end) == :empty
+  end
+
   test "max_by/2" do
     assert Enum.max_by(1..1, fn x -> :math.pow(-2, x) end) == 1
     assert Enum.max_by(1..3, fn x -> :math.pow(-2, x) end) == 2
@@ -2139,6 +2147,14 @@ defmodule EnumTest.Range do
     assert Enum.min(1..2, fn -> 0 end) === 1
   end
 
+  test "min/2 with custom sorter" do
+    sorter = fn a, b -> rem(a, 5) <= rem(b, 5) end
+    assert Enum.min(1..10, sorter) == 5
+    assert Enum.min(1..10//2, sorter) == 5
+    assert Enum.min(10..1//-1, sorter) == 10
+    assert Enum.min(1..0//1, sorter, fn -> :empty end) == :empty
+  end
+
   test "min_by/2" do
     assert Enum.min_by(1..1, fn x -> :math.pow(-2, x) end) == 1
     assert Enum.min_by(1..3, fn x -> :math.pow(-2, x) end) == 3