postcss warnings print "(Emitted value instead of an instance of Error)"

[Jessidhia]

Details

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When using a postcss plugin that may report warnings (e.g. postcss-custom-properties when it can't find the var definition, or stylelint in postcss plugin mode), the warnings are not emitted according to the webpack3+ API that requires that all warnings be instances of Error.

Error (Logs|Stacks)

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WARNING in .../node_modules/css-loader??ref--13-1!.../node_modules/postcss-loader/lib??ref--13-2!./.../style.css
(Emitted value instead of an instance of Error) stylelint: ./.../style.css:35:20: Expected a trailing semicolon (declaration-block-trailing-semicolon)
 @ ./.../style.css 2:14-159 21:1-42:3 21:157-42:2 22:19-164
 @ ./.../index.tsx
...
 @ multi (webpack)-dev-server/client?https://localhost:8080 webpack/hot/dev-server .

Reproduction [Code]

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A stylelint-as-plugin setup which config just { "extends": "stylelint-config-standard" } is probably enough to trigger warnings even on an empty CSS file.

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To clarify, the problem is that whenever a warning from postcss is printed, it is always prefixed by a warning from webpack saying (Emitted value instead of an instance of Error), although I could also add a feature request for nicely formatted errors like postcss-reporter does :)

[Nemo64]

Can confirm. This is really annoying since the log is really verbose.

[alexander-akait]

@Kovensky can you create minimum reproducible test repo? Or it is enough throw error inside postcss plugin?