Output a warning when the index kwarg is passed to #add_column.

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -606,6 +606,14 @@ def drop_table(table_name, **options)
       def add_column(table_name, column_name, type, **options)
         return if options[:if_not_exists] == true && column_exists?(table_name, column_name, type)
 
+        if options.key?(:index)
+          warn <<~WARNING
+            WARNING: #add_column ignores the :index keyword argument.
+
+            To create an index, use #add_index.
+          WARNING
+        end
+
         at = create_alter_table table_name
         at.add_column(column_name, type, **options)
         execute schema_creation.accept at

activerecord/test/cases/add_column_index_test.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+
+class AddColumnIndexTest < ActiveRecord::TestCase
+  INDEX_KWARG_WARNING = <<~WARNING
+    WARNING: #add_column ignores the :index keyword argument.
+
+    To create an index, use #add_index.
+  WARNING
+
+  setup do
+    @connection = ActiveRecord::Base.connection
+
+    @connection.create_table("add_column_tests", force: true) do |t|
+      t.string "name"
+    end
+  end
+
+  teardown do
+    @connection.drop_table "add_column_tests", if_exists: true
+  end
+
+  def test_no_index_kwarg
+    warning = capture(:stderr) do
+      @connection.add_column :add_column_tests, :height, :integer
+    end
+    assert_no_match(INDEX_KWARG_WARNING, warning)
+  end
+
+  def test_index_true_kwarg
+    warning = capture(:stderr) do
+      @connection.add_column :add_column_tests, :height, :integer, index: true
+    end
+    assert_match(INDEX_KWARG_WARNING, warning)
+  end
+
+  def test_index_false_kwarg
+    warning = capture(:stderr) do
+      @connection.add_column :add_column_tests, :height, :integer, index: false
+    end
+    assert_match(INDEX_KWARG_WARNING, warning)
+  end
+end