sass directory index import fails in some cases after #17909

[hi-ogawa]

Describe the bug

This is a regression of #17909
@userquin found an issue when testing pkg-pr-new release for vuetify https://github.com/userquin/vuetify-vite-modern-sass/

I'm still not sure if this is Vite side bug. This could be due to some inconsistency of how sass triggers custom importers.

// src/main.scss
@import "/src/test.scss";  // no issue if @import "./test.scss"

// src/test.scss
@import "./dir";

// src/dir/index.scss
.test {}

Reproduction

https://github.com/hi-ogawa/reproductions/tree/main/vite-17909-sass-import-directory-index

Steps to reproduce

For some reason, the error doesn't happen on stackblitz.

npx tiged hi-ogawa/reproductions/vite-17909-sass-import-directory-index repro
cd repro
npm i
npm run build # or dev

x Build failed in 151ms
error during build:
[vite:css] [sass] EISDIR: illegal operation on a directory, read
  ╷
1 │ @import "./dir"
  │         ^^^^^^^
  ╵
  src/test.scss 1:9  @import
  src/main.scss 5:9  root stylesheet
file: /home/hiroshi/code/personal/reproductions/repro/src/main.scss

System Info

System:
    OS: Linux 6.10 Arch Linux
    CPU: (16) x64 12th Gen Intel(R) Core(TM) i7-12650H
    Memory: 23.41 GB / 31.06 GB
    Container: Yes
    Shell: 5.2.32 - /usr/bin/bash
  Binaries:
    Node: 20.16.0 - ~/.volta/tools/image/node/20.16.0/bin/node
    npm: 10.8.1 - ~/.volta/tools/image/node/20.16.0/bin/npm
    pnpm: 9.8.0 - ~/.volta/bin/pnpm
  Browsers:
    Chromium: 128.0.6613.36
  npmPackages:
    vite: https://pkg.pr.new/vite@561b940 => 5.4.2

Used Package Manager

npm

Logs

No response

Validations

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• The provided reproduction is a minimal reproducible example of the bug.