Raise if setting a config key that is a method name

railties/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Trying to set a config key with the same name of a method now raises:
+
+    ```ruby
+    config.load_defaults = 7.0
+    # NoMethodError: Cannot assign to `load_defaults`, it is a configuration method
+    ```
+
+    *Xavier Noria*
+
 *   Deprecate `secrets:edit/show` and remove `secrets:setup`
 
     `bin/rails secrets:setup` has been deprecated since Rails 5.2 in favor of

railties/lib/rails/railtie/configuration.rb

@@ -92,9 +92,17 @@ module Rails
       end
 
     private
+      def actual_method?(key)
+        !@@options.key?(key) && respond_to?(key)
+      end
+
       def method_missing(name, *args, &blk)
         if name.end_with?("=")
-          @@options[:"#{name[0..-2]}"] = args.first
+          key = name[0..-2].to_sym
+          if actual_method?(key)
+            raise NoMethodError.new("Cannot assign to `#{key}`, it is a configuration method")
+          end
+          @@options[key] = args.first
         elsif @@options.key?(name)
           @@options[name]
         else

railties/test/application/configuration_test.rb

@@ -4371,6 +4371,17 @@ module ApplicationTests
       assert_equal true, ActiveRecord.run_after_transaction_callbacks_in_order_defined
     end
 
+    test "raises if configuration tries to assign to an actual method" do
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults = "7.0"'
+
+      error = assert_raises(NoMethodError) do
+        app "development"
+      end
+
+      assert_match(/Cannot assign to `load_defaults`, it is a configuration method/, error.message)
+    end
+
     private
       def set_custom_config(contents, config_source = "custom".inspect)
         app_file "config/custom.yml", contents

railties/test/configuration/dynamic_options_test.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+require "active_support"
+require "active_support/test_case"
+require "active_support/testing/autorun"
+require "rails/railtie/configuration"
+
+module RailtiesTest
+  class DynamicOptionsTest < ActiveSupport::TestCase
+    class Configuration < Rails::Railtie::Configuration
+      def reset_options
+        @@options = {}
+      end
+    end
+
+    setup do
+      @config = Configuration.new
+      @config.reset_options
+    end
+
+    test "arbitrary keys can be set, reset, and read" do
+      @config.foo = 1
+      assert_equal 1, @config.foo
+
+      @config.foo = 2
+      assert_equal 2, @config.foo
+    end
+
+    test "raises NoMethodError if the key is unset and the method does not exist" do
+      assert_raises(NoMethodError) do
+        @config.unset_key
+      end
+    end
+
+    test "raises NoMethodError with an informative message if assigning to an existing method" do
+      error = assert_raises(NoMethodError) do
+        @config.eager_load_namespaces = 1
+      end
+
+      assert_match(/Cannot assign to `eager_load_namespaces`, it is a configuration method/, error.message)
+    end
+  end
+end