### What?
In `app` all directories starting with `_` are now considered private
and won't be routed.
E.g. `app/_components/page.js` -> does not route
If you want to have a route starting with an underscore like
`/_settings` you can use `%5F` which is the url encode version of `_`:
`app/%5Fsettings/page.js`-> does route
### Why?
Ensures that you can create directories that will never conflict with
Next.js internal naming. E.g. when additional conventions are added for
file names.
### How?
It's a bit more involved than you might expected:
- build
- While resolving entries by traversing `app` parts with `_` are ignored
- While normalizing entires `%5F` is replaced with `_`
- I've changed one case that was relying on the entry name for resolving
to use the full path to the file instead.
- dev
- Ignored paths with underscore from being generated as part of the
initial scan of the filesystem
- Updated development and production the route matcher to replace `%5F`
with `_` so that it matches the right path.
fix NEXT-798 ([link](https://linear.app/vercel/issue/NEXT-798))
([link](https://linear.app/vercel/issue/NEXT-798))
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