refactor(compiler-sfc): bump postcss to version 8

[lstoeferle]

Since tailwindcss v2 is available, it would be great to be able to use it in Vue applications. Unfortunately it requires postcss v8+. I hope I did not break the plugins while adapting the new postcss api...

fix #2642

[edison1105]

I am also working on #2642.😭
I read this postcss8 migration article.

should we move PostCSS 8 to peerDependencies and set module.exports.postcss = true ?

[lstoeferle]

@edison1105 It seems like moving to peerDependencies does not work properly. I'll move it back to devDependencies later while keeping the module.exports.postcss = true

[edison1105]

@edison1105 It seems like moving to peerDependencies does not work properly. I'll move it back to devDependencies later while keeping the module.exports.postcss = true

both in peerDependencies and devDependencies.

  "peerDependencies": {
    ...
    "postcss": "^8.1.8"
  },

"devDependencies": {
	...
    "postcss": "^8.1.8",
  }

I'm not sure whether to do this.

[lstoeferle]

I'm not sure whether to do this.

I'm not sure if it makes sense adding it to both 🤔

[edison1105]

I'm not sure whether to do this.

I'm not sure if it makes sense adding it to both 🤔

The purpose of the two dependencies is different. In theory it is needed.

[lstoeferle]

Okay thanks for the hint 👍 I'll add it quickly to peerDependencies again. Actually it's not devDependencies 🤦 Sorry for confusion 🙃

[edison1105]

Okay thanks for the hint 👍 I'll add it quickly to peerDependencies again. Actually it's not devDependencies 🤦 Sorry for confusion 🙃

If postcss add to dependencies, there is no need to add it to peerDependencies.
add it to both devDependencies and peerDependencies and then it makes sense.

If my understanding is wrong, please correct me.

[ludofischer]

Notice that currently postcss-modules still depends on PostCSS 7, so with this change installing the Vue compiler will install PostCSS 8 AND 7, unless postcss-modules is ported first.

[songololo]

For those waiting for official v8 support for the purpose of tailwindcss, an FYI that v2 of tailwind currently has a bridging compatibility version per tailwindcss@compat, useful until above issues are resolved...

[posva]

@songololo I unlocked so you can post it 🙂 Thanks!

[yyx990803]

This seems to be based off incomplete work I was doing in the postcss-8 branch... note I was waiting on postcss-modules's new release before completing the refactor, and there was a reason why I'm not merging that branch. We also need to coordinate w/ @vue/cli before release.

See #2710