fix: i18n middleware impossible config

packages/astro/src/core/errors/errors-data.ts

@@ -1168,6 +1168,18 @@ export const MissingMiddlewareForInternationalization = {
 		"Your configuration setting `i18n.routing: 'manual'` requires you to provide your own i18n `middleware` file.",
 } satisfies ErrorData;
 
+/**
+ * @docs
+ * @description
+ * An invalid i18n middleware configuration  was detected.
+ */
+export const InvalidI18nMiddlewareConfiguration = {
+	name: 'InvalidI18nMiddlewareConfiguration',
+	title: 'Invalid internationalization middleware configuration',
+	message:
+		'The option `redirectToDefaultLocale` can be enabled only when `prefixDefaultLocale` is also set to `true`, otherwise redirects might cause infinite loops. Enable the option `prefixDefaultLocale` to continue to use `redirectToDefaultLocale`, or ensure both are set to `false`.',
+} satisfies ErrorData;
+
 /**
  * @docs
  * @description

packages/astro/src/virtual-modules/i18n.ts

@@ -3,7 +3,10 @@ import * as config from 'astro:config/server';
 import { toFallbackType } from '../core/app/common.js';
 import { toRoutingStrategy } from '../core/app/index.js';
 import type { SSRManifest } from '../core/app/types.js';
-import { IncorrectStrategyForI18n } from '../core/errors/errors-data.js';
+import {
+	IncorrectStrategyForI18n,
+	InvalidI18nMiddlewareConfiguration,
+} from '../core/errors/errors-data.js';
 import { AstroError } from '../core/errors/index.js';
 import type { RedirectToFallback } from '../i18n/index.js';
 import * as I18nInternals from '../i18n/index.js';
@@ -339,7 +342,19 @@ if (i18n?.routing === 'manual') {
 }
 
 type OnlyObject<T> = T extends object ? T : never;
-type NewAstroRoutingConfigWithoutManual = OnlyObject<NonNullable<AstroConfig['i18n']>['routing']>;
+
+export type I18nMiddlewareOptions = {
+	fallbackType: OnlyObject<NonNullable<AstroConfig['i18n']>['routing']>['fallbackType'];
+} & (
+	| {
+			prefixDefaultLocale: false;
+			redirectToDefaultLocale: false;
+	  }
+	| {
+			prefixDefaultLocale: true;
+			redirectToDefaultLocale: boolean;
+	  }
+);
 
 /**
  * @param {AstroConfig['i18n']['routing']} customOptions
@@ -371,10 +386,17 @@ type NewAstroRoutingConfigWithoutManual = OnlyObject<NonNullable<AstroConfig['i1
  *
  * ```
  */
-export let middleware: (customOptions: NewAstroRoutingConfigWithoutManual) => MiddlewareHandler;
+export let middleware: (customOptions: I18nMiddlewareOptions) => MiddlewareHandler;
 
 if (i18n?.routing === 'manual') {
-	middleware = (customOptions: NewAstroRoutingConfigWithoutManual) => {
+	middleware = (customOptions) => {
+		if (
+			customOptions.prefixDefaultLocale === false &&
+			// @ts-expect-error types do not allow this but we also check at runtime
+			customOptions.redirectToDefaultLocale === true
+		) {
+			throw new AstroError(InvalidI18nMiddlewareConfiguration);
+		}
 		strategy = toRoutingStrategy(customOptions, {});
 		fallbackType = toFallbackType(customOptions);
 		const manifest: SSRManifest['i18n'] = {

packages/astro/test/types/astro-i18n-virtual-module.ts

@@ -0,0 +1,29 @@
+import { describe, it } from 'node:test';
+import '../../client.d.ts';
+import type { I18nMiddlewareOptions } from 'astro:i18n';
+
+describe('astro:i18n', () => {
+	it('middleware', () => {
+		({
+			fallbackType: 'rewrite',
+			prefixDefaultLocale: false,
+			redirectToDefaultLocale: false,
+		}) satisfies I18nMiddlewareOptions;
+		({
+			fallbackType: 'rewrite',
+			prefixDefaultLocale: false,
+			redirectToDefaultLocale: true,
+			// @ts-expect-error invalid combination
+		}) satisfies I18nMiddlewareOptions;
+		({
+			fallbackType: 'rewrite',
+			prefixDefaultLocale: true,
+			redirectToDefaultLocale: false,
+		}) satisfies I18nMiddlewareOptions;
+		({
+			fallbackType: 'rewrite',
+			prefixDefaultLocale: true,
+			redirectToDefaultLocale: true,
+		}) satisfies I18nMiddlewareOptions;
+	});
+});