diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 28be5bea94fee..78a8da245a117 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -1,13 +1,30 @@
 # frozen_string_literal: true
 
-module Enumerable
-  INDEX_WITH_DEFAULT = Object.new
-  private_constant :INDEX_WITH_DEFAULT
+module ActiveSupport
+  module EnumerableCoreExt # :nodoc:
+    module Constants
+      private
+        def const_missing(name)
+          if name == :SoleItemExpectedError
+            ::ActiveSupport::EnumerableCoreExt::SoleItemExpectedError
+          else
+            super
+          end
+        end
+    end
+  end
+end
 
+module Enumerable
   # Error generated by +sole+ when called on an enumerable that doesn't have
   # exactly one item.
   class SoleItemExpectedError < StandardError; end
 
+  # HACK: For performance reasons, Enumerable shouldn't have any constants of its own.
+  # So we move SoleItemExpectedError into ActiveSupport::EnumerableCoreExt.
+  ActiveSupport::EnumerableCoreExt::SoleItemExpectedError = remove_const(:SoleItemExpectedError)
+  singleton_class.prepend(ActiveSupport::EnumerableCoreExt::Constants)
+
   # Enumerable#sum was added in Ruby 2.4, but it only works with Numeric elements
   # when we omit an identity.
 
@@ -106,17 +123,17 @@ def index_by
   #
   #   %i( created_at updated_at ).index_with(Time.now)
   #   # => { created_at: 2020-03-09 22:31:47, updated_at: 2020-03-09 22:31:47 }
-  def index_with(default = INDEX_WITH_DEFAULT)
+  def index_with(default = (no_default = true))
     if block_given?
       result = {}
       each { |elem| result[elem] = yield(elem) }
       result
-    elsif default != INDEX_WITH_DEFAULT
+    elsif no_default
+      to_enum(:index_with) { size if respond_to?(:size) }
+    else
       result = {}
       each { |elem| result[elem] = default }
       result
-    else
-      to_enum(:index_with) { size if respond_to?(:size) }
     end
   end
 
@@ -240,8 +257,8 @@ def in_order_of(key, series)
   def sole
     case count
     when 1   then return first # rubocop:disable Style/RedundantReturn
-    when 0   then raise SoleItemExpectedError, "no item found"
-    when 2.. then raise SoleItemExpectedError, "multiple items found"
+    when 0   then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "no item found"
+    when 2.. then raise ActiveSupport::EnumerableCoreExt::SoleItemExpectedError, "multiple items found"
     end
   end
 end
diff --git a/activesupport/test/core_ext/enumerable_test.rb b/activesupport/test/core_ext/enumerable_test.rb
index 202da47321fe9..01b9d1c879516 100644
--- a/activesupport/test/core_ext/enumerable_test.rb
+++ b/activesupport/test/core_ext/enumerable_test.rb
@@ -360,4 +360,13 @@ def test_sole
     assert_raise(expected_raise) { GenericEnumerable.new([1, 2]).sole }
     assert_raise(expected_raise) { GenericEnumerable.new([1, nil]).sole }
   end
+
+  def test_doesnt_bust_constant_cache
+    skip "Only applies to MRI" unless defined?(RubyVM.stat) && RubyVM.stat(:global_constant_state)
+
+    object = Object.new
+    assert_no_difference -> { RubyVM.stat(:global_constant_state) } do
+      object.extend(Enumerable)
+    end
+  end
 end