Method calls to Config helpers should return new objects to avoid unintended mutation

packages/metal-state/src/Config.js

@@ -1,6 +1,5 @@
 'use strict';
 
-import {object} from 'metal';
 import validators from './validators';
 
 /**
@@ -233,11 +232,12 @@ function destructShapeOfConfigs(shape) {
  */
 function mergeConfig(context, config) {
 	let obj = context;
-	if (obj === Config) {
-		obj = Object.create(Config);
-		obj.config = {};
-	}
-	object.mixin(obj.config, config);
+	const objConfig = obj.config || {};
+
+	obj = Object.create(Config);
+	obj.config = {};
+
+	Object.assign(obj.config, objConfig, config);
 	return obj;
 }
 

packages/metal-state/test/Config.js

@@ -284,4 +284,26 @@ describe('Config', function() {
 		assert.ok(config.config.validator(10));
 		assert.ok(config.config.validator('test') instanceof Error);
 	});
+
+	it('should not mutate config object with subsequent method calls', function() {
+		const config = Config.string();
+		const config2 = config.oneOf(['1', '2']);
+		const config3 = config2.required();
+
+		assert.notDeepEqual(config, config2);
+		assert.notDeepEqual(config, config3);
+		assert.notDeepEqual(config2, config3);
+
+		assert.ok(config.config.validator(1) instanceof Error);
+		assert.ok(config2.config.validator(1) instanceof Error);
+		assert.ok(config3.config.validator(1) instanceof Error);
+
+		assert.ok(config.config.validator('1'));
+		assert.ok(config2.config.validator('1'));
+		assert.ok(config3.config.validator('3') instanceof Error);
+
+		assert.isTrue(config3.config.required);
+		assert.isUndefined(config.config.required);
+		assert.isUndefined(config2.config.required);
+	});
 });