fix: App Router discovery for nested parallel-slot sub-routes

[JaredStowell]

Fix App Router route discovery for nested parallel-slot pages when the slot root has no own page, default, or intercepting route.

Before this change, a structure like:

app/inbox/page.tsx
app/inbox/@modal/profile/page.tsx

failed to produce the synthetic /inbox/profile route because @modal was filtered out too early during slot discovery.

This updates slot discovery to keep slots that only contain descendant page files, so the existing synthetic sub-route generation can create and match routes like /inbox/profile.

Changes

• treat nested descendant page files as enough for a parallel slot to participate in routing
• keep the existing behavior for slot roots with page, default, or intercepting routes
• add a routing regression test covering:
  • route discovery of /inbox/profile
  • successful matchAppRoute('/inbox/profile', routes)
  • correct slot page selection for the synthetic route

Testing

• pnpm test tests/routing.test.ts
• pnpm test tests/app-router.test.ts
• pnpm run fmt
• pnpm run typecheck

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commit: 4897b1f

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: a4070516a9

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[james-elicx]

/bigbonk review

[ask-bonk]

Review

The net code change here is a small defensive fix: before this PR, discoverSlotSubRoutes would create synthetic routes with pagePath: null (set from childrenDefault) when the parent directory had no default.tsx. This would produce routes that fail at render time with a "Page has no default export" error. The fix correctly gates synthetic sub-route creation on the existence of childrenDefault.

The change is correct and the test is well-structured. Two items worth noting:

PR description is misleading

The PR description says:

This updates slot discovery to keep slots that only contain descendant page files

This described the first commit (a407051), but the second commit (a1d4c02) reverted that change entirely. The actual net effect of this PR is the opposite of what the description says: it adds a guard to skip synthetic sub-route creation when there is no childrenDefault, rather than expanding slot discovery. The discoverParallelSlots filter at line 650 is unchanged.

Please update the PR title/description to match the actual change — something like "fix: skip synthetic slot sub-routes when parent has no default.tsx fallback".

The original problem from the PR description remains unsolved

The scenario described in the PR body:

app/inbox/page.tsx
app/inbox/@modal/profile/page.tsx

(where @modal has no root page.tsx or default.tsx) still does not produce a /inbox/profile route after this PR. The discoverParallelSlots filter at line 650 still excludes @modal from parallelSlots, so discoverSlotSubRoutes never sees its nested pages. This PR only fixes the downstream symptom (creating broken synthetic routes with null pagePath), not the discovery gap.

If the intent is to also support the "slot with only nested descendants" case, that would need a separate follow-up to relax the discoverParallelSlots filter — which was the original first commit that got reverted. That's fine as a separate PR, but worth tracking.

[ask-bonk]

This is the right fix — previously, if childrenDefault was null, synthetic routes were created with pagePath: null, which would 500 at render time. Combining the two guards into one continue is clean.

Minor: the original code had the subPathMap.size === 0 check first (cheap) before the findFile call (does filesystem I/O). The new code always calls findFile even when subPathMap is empty. Consider preserving the short-circuit:

Suggested change

	if (subPathMap.size === 0 || !childrenDefault) continue;
	if (subPathMap.size === 0) continue;
	// Find the default.tsx for the children slot at the parent directory
	const childrenDefault = findFile(parentPageDir, "default", matcher);
	if (!childrenDefault) continue;

This avoids a pointless findFile call when there are no sub-paths. Not a big deal since fs.existsSync is fast, but it's a free optimization and arguably clearer about the two independent exit conditions.

[ask-bonk]

Good test. The name accurately describes what it asserts.

Note that this test gives @modal a default.tsx (line 484), so the slot passes the discoverParallelSlots filter. What's being tested is purely the childrenDefault guard in discoverSlotSubRoutes.

It might be worth adding a companion test for the other case: @modal with no root page.tsx/default.tsx and only @modal/profile/page.tsx — which tests that discoverParallelSlots itself excludes such a slot. That would document the current behavior and guard against future regressions if someone relaxes the filter.

[ask-bonk]

Review submitted. Here's a summary of the feedback:

The code change is correct — it fixes a real bug where synthetic slot sub-routes were created with pagePath: null when the parent directory had no default.tsx, leading to 500 errors at render time.

Key feedback points:

1. PR description is misleading — The description says it "keeps slots that only contain descendant page files," which described the first commit that was then reverted. The actual net change is the opposite: it adds a guard to skip synthetic route creation when childrenDefault is missing. I asked the author to update the title/description.

2. Minor optimization — The combined guard if (subPathMap.size === 0 || !childrenDefault) continue always calls findFile even when subPathMap is empty. Suggested keeping the original short-circuit pattern with two separate guards.

3. The original problem remains unsolved — The scenario in the PR body (@modal with only nested descendants and no root page/default) still doesn't produce routes, because discoverParallelSlots still filters out such slots at line 650. Flagged this as needing a follow-up.

4. Test suggestion — Recommended adding a companion test for the case where @modal has no root page.tsx/default.tsx at all (not just missing childrenDefault), to document the discoverParallelSlots filter behavior.

github run