Warn when the site has a pathname but not using base

packages/astro/src/core/config/schema.ts

@@ -323,11 +323,23 @@ export function createRelativeSchema(cmd: string, fileProtocolRoot: URL) {
 			config.build.client = new URL('./dist/client/', config.outDir);
 		}
 		const trimmedBase = trimSlashes(config.base);
-		if (trimmedBase.length && config.trailingSlash === 'never') {
-			config.base = prependForwardSlash(trimmedBase);
-		} else {
-			config.base = prependForwardSlash(appendForwardSlash(trimmedBase));
+
+		// If there is no base but there is a base for site config, warn.
+		const sitePathname = config.site && new URL(config.site).pathname;
+		if(trimmedBase.length && sitePathname && sitePathname !== '/') {
+			config.base = sitePathname;
+			/* eslint-disable no-console */
+			console.warn(`The site configuration value includes a pathname of ${sitePathname} but there is no base configuration.
+			
+A future version of Astro will stop using the site pathname when producing <link> and <script> tags. Set your site's base with the base configuration.`)
 		}
+
+		if(trimmedBase.length && config.trailingSlash === 'never') {
+      config.base = prependForwardSlash(trimmedBase);
+    } else {
+      config.base = prependForwardSlash(appendForwardSlash(trimmedBase));
+    }
+
 		return config;
 	});