Repository: create-react-app
Issue: Warnings are treated as errors
Labels: needs triage,issue: bug report

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### Describe the bug

Any warning is treated as an error, and will make compilation fail. This is especially annoying for eslint warnings, because something you just want to quickly try something without having to conform to all the rules right away. In that case, the errors (which are supposed to be warnings!) are only in the way.

### Did you try recovering your dependencies?

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I'm not going to delete `package-lock.json`. It's there for a reason. But if I do it temporarily and restore it afterwards, half an hour later I still have the same problem. And everyone in my team does as well.

### Which terms did you search for in User Guide?

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"errors" "warnings" "eslint" that sort of things.
Found an especially promising issue, #2453.
This suggests to set `CI=false`, which doesn't help at all. Looking at the source code, the problem described in #2453 specifically shouldn't occur if `CI` is not set (`undefined` at compile-time), which it is for us.

### Environment

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```
npx: installed 67 in 5.049s

Environment Info:

  current version of create-react-app: 4.0.3
  running from C:\Users\martijn.saly\AppData\Roaming\npm-cache\_npx\25308\node_modules\create-react-app

  System:
    OS: Windows 10 10.0.19043
    CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Binaries:
    Node: 14.17.1 - C:\Program Files\nodejs\node.EXE
    Yarn: Not Found
    npm: 6.14.13 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 91.0.4472.114
    Edge: Spartan (44.19041.1023.0), Chromium (91.0.864.54)
    Internet Explorer: 11.0.19041.1
  npmPackages:
    react: 17.0.2 => 17.0.2
    react-dom: 17.0.2 => 17.0.2
    react-scripts: 4.0.3 => 4.0.3
  npmGlobalPackages:
    create-react-app: Not Found
```

("Browsers" is a lie)

### Steps to reproduce

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1. Make an ESLint warning, like creating a variable and then never using it.
2. Compile (or just save, and let compilation occur by itself).
3. Observe the website in front of your face, and see it go "red" completely.

### Expected behavior

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No error, especially not one that overlaps the whole site and blocks everything including compilation. Just a warning in the console will suffice.

### Actual behavior

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An error that is useless at the time of just working on code, which completely blocks compilation, and hinders from just having a bit of a fiddle, because the bloody website is completely gone when just having an unused variable! Who bloody cares about that NOW, we care about that when making a production build!

![image](https://user-images.githubusercontent.com/152227/122775872-de40e400-d2aa-11eb-8d23-f1a86809afdb.png)


### Reproducible demo

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Sorry but it's really way too obvious for that. Also it's been discussed before in the issue I linked. There was just no proper solution, that's all.

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