No fallback to index.js when importing a directory using an alias

[charlesmass2]

Describe the bug

Importing a directory using import won't fallback to the index.js file anymore.
Get the following error:

I created a very simple repo for an easy reproduction.

I noticed that the issue started at version 5.1.0, more specifically with version 5.1.0-beta.4.

Reproduction

https://github.com/charlesmass2/vite-index-fallback-fail

Steps to reproduce

Run npm install followed by npm run dev

System Info

System:
    OS: Linux 6.5 Ubuntu 22.04.2 LTS 22.04.2 LTS (Jammy Jellyfish)
    CPU: (20) x64 13th Gen Intel(R) Core(TM) i7-13700H
    Memory: 7.28 GB / 31.05 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 20.3.1 - /usr/bin/node
    npm: 9.6.7 - /usr/bin/npm
  Browsers:
    Chrome: 121.0.6167.139
  npmPackages:
    @vitejs/plugin-vue: ^5.0.3 => 5.0.3 
    vite: 5.1.0 => 5.1.0

This is also reproduced in Alpine docker container.

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
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• The provided reproduction is a minimal reproducible example of the bug.

[hi-ogawa]

It looks like newly introduced server.fs.cachedChecks (#15279) have some inconsistency when handling absolute path with trailing slash. Thanks for the reproduction btw, I updated it to compare several cases:

https://stackblitz.com/edit/github-d5e7nn?file=src%2Fmain.js

// fails
import * as lib1 from '@/utils/'; // @ alias is rewritten to absolute path
console.log({ lib1 });

// others all work
import * as lib2 from '@/utils';
console.log({ lib2 });

import * as lib3 from '@/utils/index.js';
console.log({ lib3 });

 // for relative path, `path.resolve` (which removes trailing slash)  happens before reaching fs caching, so this doesn't manifest the issue
import * as lib4 from './utils/';
console.log({ lib4 });

import * as lib5 from './utils';
console.log({ lib5 });

import * as lib6 from './utils///././//'; // still normalized to the same "utils"
console.log({ lib6 });

Quick workaround would be to disable server.fs.cachedChecks by:

export default defineConfig({
  server: {
    fs: {
      cachedChecks: false,
    },
  },
});

I think non-cached version (e.g plain NodeJS's fs.statSync) normalizes path automatically (e.g. relative path, . and .., trailing slash removal etc... probably equivalent to path.resolve?), but cached version is missing trailing slash removal.

[charlesmass2]

I can confirm that disabling cachedChecks works. Good workaround in my case, until this minor issue is resolved.
Thanks for the quick reply !