Remove default fallback behavior when route group is missing a default

[ztanner]

This test case was added in #59752 but this doesn't seem like the correct behavior.

The original PR was intended to be smart about resolving /default.tsx to a route group default (eg /(foo)/default.tsx) when one wasn't specified. But since the route group is creating a new hierarchy in the tree and defines its own layout, if the route group layout doesn't specify a default, then the not found behavior seems correct.

To fix unexpected not-found behavior in this case, you should specify a default at the same level as the layout where the missing slot(s) might be rendered.

Closes NEXT-2565

[ztanner]

• Remove default fallback behavior when route group is missing a default #62370 👈
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[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
buildDuration	14.4s	14.5s	N/A
buildDurationCached	8s	6.8s	N/A
nodeModulesSize	196 MB	196 MB	N/A
nextStartRea..uration (ms)	404ms	407ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
1068-HASH.js gzip	30.3 kB	30.3 kB	N/A
3f784ff6-HASH.js gzip	53.5 kB	53.5 kB	N/A
4944-HASH.js gzip	5.04 kB	5.03 kB	N/A
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webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	47.1 kB	47.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

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_app-HASH.js gzip	196 B	196 B	✓
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amp-HASH.js gzip	503 B	504 B	N/A
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Overall change	931 B	931 B	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
_buildManifest.js gzip	485 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
index.html gzip	528 B	528 B	✓
link.html gzip	539 B	540 B	N/A
withRouter.html gzip	523 B	522 B	N/A
Overall change	528 B	528 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
edge-ssr.js gzip	2.3 kB	2.3 kB	N/A
page.js gzip	2.99 kB	2.99 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
middleware-b..fest.js gzip	626 B	627 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	466 B	465 B	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	990 B	990 B	✓

Next Runtimes

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app-page-exp...dev.js gzip	167 kB	167 kB	✓
app-page-exp..prod.js gzip	96.2 kB	96.2 kB	✓
app-page-tur..prod.js gzip	97.9 kB	97.9 kB	✓
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app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
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app-route.ru...dev.js gzip	21.8 kB	21.8 kB	✓
app-route.ru..prod.js gzip	14.7 kB	14.7 kB	✓
pages-api-tu..prod.js gzip	9.49 kB	9.49 kB	✓
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pages-api.ru..prod.js gzip	9.48 kB	9.48 kB	✓
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pages.runtim..prod.js gzip	22.3 kB	22.3 kB	✓
server.runti..prod.js gzip	50.4 kB	50.4 kB	✓
Overall change	930 kB	930 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js 02-21-Remove_default_fallback_behavior_when_route_group_is_missing_a_default	Change
0.pack gzip	1.49 MB	1.49 MB	⚠️ +517 B
index.pack gzip	104 kB	104 kB	⚠️ +331 B
Overall change	1.59 MB	1.6 MB	⚠️ +848 B

Commit: a4c5f8b

[ijjk]

Tests Passed