### What?
When relying on a `default` route as a fallback, greedier catch-all
segments in the application hierarchy would take precedence, causing
unexpected errors/matching behavior.
### Why?
When performing parallel route catch-all normalization, we push
potential catch-all matches to paths without considering that a path
might instead be matched by a `default` page. Because of this, the
catch-all take precedence and the app will not try and load the default.
For example, given this structure:
```
{
"/": ["/page"],
"/[[...catchAll]]": ["/[[...catchAll]]/page"],
"/nested/[foo]/[bar]": ["/nested/[foo]/[bar]/@slot/page"],
"/nested/[foo]/[bar]/[baz]": ["/nested/[foo]/[bar]/@slot/[baz]/page"],
}
```
(Where there's a `/nested/[foo]/[bar]/default.tsx`)
The route normalization logic would produce:
```
{
"/": ["/page"],
"/[[...catchAll]]": ["/[[...catchAll]]/page"],
"/nested/[foo]/[bar]": [
"/nested/[foo]/[bar]/@slot/page",
"/[[...catchAll]]/page",
],
"/nested/[foo]/[bar]/[baz]": [
"/nested/[foo]/[bar]/@slot/[baz]/page",
"/[[...catchAll]]/page",
],
}
```
This means that when building the `LoaderTree`, it won't ever try to
find the default for that segment. **This solution operates under the
assumption that if you defined a `default` at a particular layout
segment, you intend for that to render in place of a greedier
catch-all.** (Let me know if this is an incorrect assumption)
### How?
We can't safely normalize catch-all parallel routes without having
context about where the `default` segments are, so this updates
`appPaths` to be inclusive of default segments and then filters them
when doing anything relating to build/export to maintain existing
behavior. We use this information to check if an existing default exists
at the same segment level that we'd push the catch-all to. If one
exists, we don't push the catch-all. Otherwise we proceed as normal.
Closes NEXT-1987
90f95399dd