Auto-configure cors on WelcomePageHandlerMapping

Update `WebMvcAutoConfiguration` to automatically apply cors
configuration to the `WelcomePageHandlerMapping`.

Fixes gh-21048

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -451,6 +451,7 @@ public class WebMvcAutoConfiguration {
 					new TemplateAvailabilityProviders(applicationContext), applicationContext, getWelcomePage(),
 					this.mvcProperties.getStaticPathPattern());
 			welcomePageHandlerMapping.setInterceptors(getInterceptors());
+			welcomePageHandlerMapping.setCorsConfigurations(getCorsConfigurations());
 			return welcomePageHandlerMapping;
 		}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfigurationTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -72,6 +72,7 @@ import org.springframework.web.accept.ParameterContentNegotiationStrategy;
 import org.springframework.web.accept.PathExtensionContentNegotiationStrategy;
 import org.springframework.web.bind.support.ConfigurableWebBindingInitializer;
 import org.springframework.web.context.request.ServletWebRequest;
+import org.springframework.web.cors.UrlBasedCorsConfigurationSource;
 import org.springframework.web.filter.FormContentFilter;
 import org.springframework.web.filter.HiddenHttpMethodFilter;
 import org.springframework.web.filter.RequestContextFilter;
@@ -83,6 +84,7 @@ import org.springframework.web.servlet.View;
 import org.springframework.web.servlet.ViewResolver;
 import org.springframework.web.servlet.config.annotation.AsyncSupportConfigurer;
 import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
+import org.springframework.web.servlet.config.annotation.CorsRegistry;
 import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver;
@@ -554,7 +556,19 @@ public class WebMvcAutoConfigurationTests {
 		this.contextRunner.withPropertyValues("spring.resources.static-locations:classpath:/welcome-page/")
 				.run((context) -> {
 					assertThat(context).hasSingleBean(WelcomePageHandlerMapping.class);
-					assertThat(context.getBean(WelcomePageHandlerMapping.class).getRootHandler()).isNotNull();
+					WelcomePageHandlerMapping bean = context.getBean(WelcomePageHandlerMapping.class);
+					assertThat(bean.getRootHandler()).isNotNull();
+				});
+	}
+
+	@Test
+	public void welcomePageHandlerIncludesCorsConfiguration() {
+		this.contextRunner.withPropertyValues("spring.resources.static-locations:classpath:/welcome-page/")
+				.withUserConfiguration(CorsConfigurer.class).run((context) -> {
+					WelcomePageHandlerMapping bean = context.getBean(WelcomePageHandlerMapping.class);
+					UrlBasedCorsConfigurationSource source = (UrlBasedCorsConfigurationSource) ReflectionTestUtils
+							.getField(bean, "corsConfigurationSource");
+					assertThat(source.getCorsConfigurations()).containsKey("/**");
 				});
 	}
 
@@ -1096,4 +1110,14 @@ public class WebMvcAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	static class CorsConfigurer implements WebMvcConfigurer {
+
+		@Override
+		public void addCorsMappings(CorsRegistry registry) {
+			registry.addMapping("/**").allowedMethods("GET");
+		}
+
+	}
+
 }