* Ensure preload works correctly with locale option

* Fix conditional import

* Update size-limit tests

* DCE some code

* Resolve merge error
Co-authored-by: NTim Neutkens <timneutkens@me.com>

This updates the documentation with the new `redirect` and `notFound` return values adding `redirect` to the `getStaticProps` section and adding a new section describing the return values allowed for `getServerSideProps`

Closes: https://github.com/vercel/next.js/pull/18626
Closes: https://github.com/vercel/next.js/issues/18691

Fixes #18643

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Hi next.js team!
Was trying out the canary and came across the error!

The redirect to https://err.sh was to
`https://github.com/vercel/nextjs`, not
`https://github.com/vercel/next.js`

In PR #18147, we fixed the default loader to prevent upscaling images.

This PR fixes the same bug for 3rd party loaders.

---

Fixes #18648

Older versions of Safari (and other browsers) do not support webp format so we were incorrectly falling back to png.

This PR fixes #18509 so that we fallback to the original image format if a modern format like webp was not explicitly provided in the Accept header.

Tests were added to ensure that the Accept header for Safari, Firefox, and Chrome work properly.

Our code uses `core-js` to polyfill and because core-js [`es.symbol.description`](https://github.com/zloirock/core-js/blob/master/packages/core-js/modules/es.symbol.description.js#L14-L16) doesn't just check for `Symbol.prototype.description`, it will try to polyfill again but fail to do so because Next.js polyfill is not configurable. By default, `Object.defineProperty` set `configurable` to `false`. 

Here are the screenshots of the error on Edge 44.18362.449.0 using Next.js v10.0.1-canary.1
![Screen Shot 2020-10-30 at 9 44 05 PM](https://user-images.githubusercontent.com/1637627/97771409-da810700-1af9-11eb-8900-fe49d718dc94.png)

![Screen Shot 2020-10-30 at 9 51 43 PM](https://user-images.githubusercontent.com/1637627/97771433-321f7280-1afa-11eb-8928-fa4d902a01dd.png)

 

This makes sure we don't use invalid `x-now-route-matches` which can occur when `i18n` default locale is visited and a prerendered page is matched. To correct this we first check if we are able to derive the correct params from the URL and then bail on parsing `x-now-route-matches` if we are. Additional test cases are being added in the builder to ensure we don't regress on this

x-ref: https://github.com/vercel/next.js/discussions/18443
Fixes #18602

Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

This PR deprecates the `unsized` property from NextImage because the property did not accomplish the desired effect.

Users should rely on one of the new layouts instead:

- `<Image layout="fixed" />`
- `<Image layout="intrinsic" />`
- `<Image layout="responsive" />`
- `<Image layout="fill" />`

The `unsized` property will continue to work as-is in production but is deprecated and will throw in dev.

---

### TODO:
- [x] test `layout=fill` in typescript types
- [x] test `layout=fill` render behavior
- [x] test that `unsized` switches to `layout=fill`
- [x] test `next dev` erroring on `unsized`
- [ ] layout docs (tracked in issue #18554)
- [ ] both `layout=fill` and `layout=responsive` use all deviceWidths in the srcset

---

Fixes #18541 
Co-authored-by: NSteven <steven@ceriously.com>

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

* small tweaks for those testing on an existing site with missing author or featured image on posts

When migrating WordPress or using an exisitng site, sometimes you have published posts wth missing data. The graphql plugin will properly resolve these to null, but some of existing code accessing tries to access the node property on these null values. I've made these properties optional in the pages and also tweaked some of the components along similar lines.

* Lint fix
Co-authored-by: NLuis Alvarez <luis@vercel.com>
Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Fixes #18523

## description
The i18n-routing Vercel deploy button URLs was:
https://vercel.com/import/project?template=https://github.com/vercel/next.js/tree/canary/examples/amp

So, I fixed the URLs for examples/i18n-routing:
https://vercel.com/import/project?template=https://github.com/vercel/next.js/tree/canary/examples/i18n-routing

Co-authored-by: NLuis Alvarez D. <luis@vercel.com>

This PR introduces a new `layout` property.

This allows 3 possible values (`fixed`, `intrinsic`, or `responsive`) which solve many use cases we have seen since 10.0.0 and will hopefully avoid usage of `unsized`.

Fixes #18351 
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

This ensures we provide the current `locale`, `locales`, and `defaultLocale` in the context when rendering the 404 for a blocking SSG page

Fixes: https://github.com/vercel/next.js/issues/18505

- Update Next.js version to 10.0.0
- Update the other dependencies
- Modify `next.config.js` to prevent circular `__self` and `__source`in dev env (ref: https://github.com/developit/nextjs-preact-demo/issues/25)

This makes sure custom-routes function properly with i18n support. Since the `locale` is stripped from the `pathname` this re-adds when checking custom-routes to allow the locale to be matched against

Closes: https://github.com/vercel/next.js/issues/18349

Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

This adds an err.sh with info that can help when tracking down errors from invalid i18n config in next.config.js

Firefox 61:
<img width="513" alt="Screen Shot 2020-10-28 at 5 28 30 PM" src="https://user-images.githubusercontent.com/105127/97498751-1a51be00-1943-11eb-977a-832123363cef.png">

Chrome 61:
<img width="548" alt="Screen Shot 2020-10-28 at 5 26 55 PM" src="https://user-images.githubusercontent.com/105127/97498755-1c1b8180-1943-11eb-8423-bc4e044a1b00.png">

Safari 11:
<img width="477" alt="Screen Shot 2020-10-28 at 5 23 06 PM" src="https://user-images.githubusercontent.com/105127/97498761-1d4cae80-1943-11eb-9de2-e6b26644110a.png">

Fixes #17825 

Closes #17626

* Add err.sh for image config errors

* Apply suggestions from code review
Co-authored-by: NSteven <steven@ceriously.com>
Co-authored-by: NSteven <steven@ceriously.com>

This PR documents some details that are mentioned in the blog post https://nextjs.org/blog/next-10

This PR explains the caching algorithm for the default loader.

* Add err.sh for missing images domain

* Apply suggestions from code review
Co-authored-by: NSteven <steven@ceriously.com>

* Update test
Co-authored-by: NSteven <steven@ceriously.com>