I noticed this problem only in my SvelteKit stories. It works correctly with React stories.

• @storybook/{svelte,sveltekit,*} 7.0.0-rc.3
• @sveltejs/kit 1.5.0
• react 18.2.0
• unplugin-icons 0.15.2
• vite 4.1.1

I was able to workaround the issue by monkey-patching svelte-docgen-plugin's transform, excluding any resources whose IDs start with ~icons/:

const workaroundSvelteDocgenPluginConflictWithUnpluginIcons = (config: InlineConfig) => {
  if (!config.plugins) return config

  const [_internalPlugins, ...userPlugins] = config.plugins as Plugin[]
  const docgenPlugin = userPlugins.find(plugin => plugin.name === 'storybook:svelte-docgen-plugin')
  if (docgenPlugin) {
    const origTransform = docgenPlugin.transform
    const newTransform: typeof origTransform = (code, id, options) => {
      // ignore unplugin-icons resources
      if (id.startsWith('~icons/')) {
        return
      }
      return (origTransform as Function)?.call(docgenPlugin, code, id, options)
    }
    docgenPlugin.transform = newTransform
    docgenPlugin.enforce = 'post'
  }
  return config
}

// .storybook/main.ts
import type { Plugin, InlineConfig } from 'vite'
import type { StorybookConfig } from '@storybook/sveltekit'

// https://github.com/storybookjs/storybook/issues/20562
const workaroundSvelteDocgenPluginConflictWithUnpluginIcons = (config: InlineConfig) => {
  if (!config.plugins) return config

  const [_internalPlugins, ...userPlugins] = config.plugins as Plugin[]
  const docgenPlugin = userPlugins.find(plugin => plugin.name === 'storybook:svelte-docgen-plugin')
  if (docgenPlugin) {
    const origTransform = docgenPlugin.transform
    const newTransform: typeof origTransform = (code, id, options) => {
      if (id.startsWith('~icons/')) {
        return
      }
      return (origTransform as Function)?.call(docgenPlugin, code, id, options)
    }
    docgenPlugin.transform = newTransform
    docgenPlugin.enforce = 'post'
  }
  return config
}

const config: StorybookConfig = {
  stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|ts|tsx)'],
  addons: [
    '@storybook/addon-links',
    '@storybook/addon-essentials',
    '@storybook/addon-interactions',
  ],
  framework: {
    name: '@storybook/sveltekit',
    options: {},
  },
  docs: {
    autodocs: 'tag',
  },
  viteFinal(config) {
    return workaroundSvelteDocgenPluginConflictWithUnpluginIcons(config)
  }
}
export default config

// .storybook/main.js
import { mergeConfig } from "vite";
import Icons from "unplugin-icons/vite";
import path from "path";

// https://github.com/storybookjs/storybook/issues/20562
/**
 * @param {import('vite').InlineConfig} config
 */
const workaroundSvelteDocgenPluginConflictWithUnpluginIcons = (config) => {
  if (!config.plugins) return config

  const [_internalPlugins, ...userPlugins] = config.plugins
  const docgenPlugin = userPlugins.find(plugin => plugin.name === 'storybook:svelte-docgen-plugin')
  if (docgenPlugin) {
    const origTransform = docgenPlugin.transform
    const newTransform = (code, id, options) => {
      if (id.startsWith('~icons/')) {
        return
      }
      return origTransform?.call(docgenPlugin, code, id, options)
    }
    docgenPlugin.transform = newTransform
    docgenPlugin.enforce = 'post'
  }
  return config
}

/**
 * @type {import('@storybook/sveltekit').StorybookConfig}
 */
const config = {
  stories: ["../src/**/*.mdx", "../src/**/*.stories.@(js|ts|svelte)"],
  addons: [
    "@storybook/addon-links",
    "@storybook/addon-essentials",
    "@storybook/addon-interactions",
  ],
  framework: {
    name: "@storybook/sveltekit",
    options: {},
  },
  docs: {
    autodocs: "tag",
  },
  async viteFinal(config) {
    config = workaroundSvelteDocgenPluginConflictWithUnpluginIcons(config)

    // Merge custom configuration into the default config
    return mergeConfig(config, {
      plugins: [
        Icons({
          compiler: "svelte",
        }),
      ],
      // Add dependencies to pre-optimization
      optimizeDeps: {
        include: ["storybook-dark-mode"],
      },
    });
  },
};

export default config;

Thanks for the workaround, @tony19. I'm reluctant to do this in the base plugin, since technically anyone could use ~icons/ for anything, and they may be components that we need to process. I've opened unplugin/unplugin-icons#274 to ask whether there's something that should change on unplugin-icons side, in order to prevent other plugins like ours from processing these virtual files.

I had to add "types": ["unplugin-icons/types/svelte"] to my tsconfig in order to get everything to compile properly. See if this avoids the need for a workaround.

It looks to me like this would only be hit when a preprocessor is being used:

I'm not quite sure what this code is doing. It looks like the src is passed in, but rather than using it the file is read from the file system to get the src again. I don't know why though. Any ideas @IanVS ?

No, I think it just needs to be changed not to try to read from disk. I meant to make that change but then forgot about it. Thanks for the reminder. If anyone wants to take a crack at it before I get to it, feel free!

Does anyone have a solution for this issue?

Yes, I believe the solution is to change the svelte-docgen plugin in storybook to read the src instead of reading the contents from disk. Would you be willing to open a PR, @ryu-man?

Workaround until this gets fixed: yarn patch @storybook/svelte-vite. Then, edit dist/preset.js and replace:

if(docPreprocessOptions){let rawSource=import_fs.default.readFileSync(resource).toString(),{code:fileContent}=await(0,import_compiler.preprocess)(rawSource,docPreprocessOptions,{filename:resource});docOptions={fileContent}}

with

if(docPreprocessOptions)try{let rawSource=import_fs.default.readFileSync(resource).toString(),{code:fileContent}=await(0,import_compiler.preprocess)(rawSource,docPreprocessOptions,{filename:resource});docOptions={fileContent}}catch(e){} else

Notice right after the if(docPreprocessOptions), add just the keyword try right before the opening bracket, then right after the closing bracket before the else add catch(e){}.

@IanVS I opened a PR for this (#26838)