Use Integer#div instead of Integer#/ to avoid side effects from mathn

activesupport/lib/active_support/core_ext/array/grouping.rb

@@ -58,7 +58,7 @@ class Array
     # size / number gives minor group size;
     # size % number gives how many objects need extra accommodation;
     # each group hold either division or division + 1 items.
-    division = size / number
+    division = size.div number
     modulo = size % number
 
     # create a new array avoiding dup

activesupport/test/core_ext/array_ext_test.rb

@@ -112,6 +112,14 @@ class ArrayExtToSTests < ActiveSupport::TestCase
 end
 
 class ArrayExtGroupingTests < ActiveSupport::TestCase
+  def setup
+    Fixnum.send :private, :/  # test we avoid Integer#/ (redefined by mathn)
+  end
+
+  def teardown
+    Fixnum.send :public, :/
+  end
+
   def test_in_groups_of_with_perfect_fit
     groups = []
     ('a'..'i').to_a.in_groups_of(3) do |group|