fix(build): enable prerendering of interception routes with generateStaticParams

Fixes a bug where interception routes in parallel slots could not be prerendered
using generateStaticParams. The previous implementation only examined "children"
segments when building static paths, missing segments from parallel routes that
contribute to the pathname.

Introduces extractPathnameSegments() which:
- Traverses the entire loader tree (children + parallel routes) using BFS
- Correctly tracks URL depth (skipping route groups/parallel markers, including
  interception markers which are actual URL components)
- Validates that static segment prefixes match the target pathname
- Returns ALL segments that contribute to pathname construction

This enables routes like app/@modal/(.)photo/[id] to be prerendered when they
export generateStaticParams, fixing 404 responses that occurred when these
routes were accessed directly.

Author: wyattjoh

packages/next/src/build/segment-config/app/app-segments.ts

@@ -12,14 +12,15 @@ import {
   isAppPageRouteModule,
 } from '../../../server/route-modules/checks'
 import { isClientReference } from '../../../lib/client-and-server-references'
-import { getSegmentParam } from '../../../shared/lib/router/utils/get-segment-param'
+import {
+  getSegmentParam,
+  type SegmentParam,
+} from '../../../shared/lib/router/utils/get-segment-param'
 import {
   getLayoutOrPageModule,
   type LoaderTree,
 } from '../../../server/lib/app-dir-module'
 import { PAGE_SEGMENT_KEY } from '../../../shared/lib/segment'
-import type { FallbackRouteParam } from '../../static-paths/types'
-import { createFallbackRouteParam } from '../../static-paths/utils'
 import type { DynamicParamTypes } from '../../../shared/lib/app-router-types'
 
 type GenerateStaticParams = (options: { params?: Params }) => Promise<Params[]>
@@ -223,17 +224,17 @@ export function collectSegments(
 }
 
 /**
- * Collects the fallback route params for a given app page route module. This is
- * a variant of the `collectSegments` function that only collects the fallback
- * route params without importing anything.
+ * Collects the segment params for a given app page route module. This is
+ * a variant of the `collectSegments` function that only collects the segment
+ * params without importing anything.
  *
  * @param routeModule the app page route module
- * @returns the fallback route params for the app page route module
+ * @returns the segment params for the app page route module
  */
-export function collectFallbackRouteParams(
+export function collectSegmentParams(
   routeModule: AppPageRouteModule
-): readonly FallbackRouteParam[] {
-  const uniqueSegments = new Map<string, FallbackRouteParam>()
+): readonly SegmentParam[] {
+  const uniqueSegments = new Map<string, SegmentParam>()
 
   // Queue will store tuples of [loaderTree, isParallelRouteSegment]
   type QueueItem = [loaderTree: LoaderTree, isParallelRouteSegment: boolean]
@@ -248,14 +249,7 @@ export function collectFallbackRouteParams(
     if (segmentParam) {
       const key = `${name}-${segmentParam.param}-${isParallelRouteSegment ? 'pr' : 'np'}`
       if (!uniqueSegments.has(key)) {
-        uniqueSegments.set(
-          key,
-          createFallbackRouteParam(
-            segmentParam.param,
-            segmentParam.type,
-            isParallelRouteSegment
-          )
-        )
+        uniqueSegments.set(key, segmentParam)
       }
     }