Remove default fallback behavior when route group is missing a default

packages/next/src/build/webpack/loaders/next-app-loader.ts

@@ -450,22 +450,12 @@ async function createTreeCodeFromPath(
           adjacentParallelSegment === 'children'
             ? ''
             : `/${adjacentParallelSegment}`
-        let defaultPath = await resolver(
-          `${appDirPrefix}${segmentPath}${actualSegment}/default`
-        )
 
-        if (!defaultPath) {
-          // no default was found at this segment. Check if the normalized segment resolves a default
-          // for example: /(level1)/(level2)/default doesn't exist, but /default does
-          const normalizedDefault = await resolver(
-            `${appDirPrefix}${normalizeAppPath(
-              segmentPath
-            )}/${actualSegment}/default`
-          )
-
-          // if a default is found, use that. Otherwise use the fallback, which will trigger a `notFound()`
-          defaultPath = normalizedDefault ?? PARALLEL_ROUTE_DEFAULT_PATH
-        }
+        // if a default is found, use that. Otherwise use the fallback, which will trigger a `notFound()`
+        const defaultPath =
+          (await resolver(
+            `${appDirPrefix}${segmentPath}${actualSegment}/default`
+          )) ?? PARALLEL_ROUTE_DEFAULT_PATH
 
         nestedCollectedAsyncImports.push(defaultPath)
         props[normalizeParallelKey(adjacentParallelSegment)] = `[

test/turbopack-tests-manifest.json

@@ -3563,18 +3563,6 @@
     "flakey": [],
     "runtimeError": false
   },
-  "test/e2e/app-dir/interception-route-groups/interception-route-groups.test.ts": {
-    "passed": [
-      "interception route groups (no default) should use the default fallback (a 404) if there is no custom default page"
-    ],
-    "failed": [
-      "interception route groups (with default) should render the root default when a route group doesn't have a default",
-      "interception route groups (with default) should work when nested a level deeper"
-    ],
-    "pending": [],
-    "flakey": [],
-    "runtimeError": false
-  },
   "test/e2e/app-dir/interception-route-prefetch-cache/interception-route-prefetch-cache.test.ts": {
     "passed": [
       "interception-route-prefetch-cache runtime = edge should render the correct interception when two distinct layouts share the same path structure",