Build fails when CSS contains escaped space

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/github/vercel/next.js/tree/canary/examples/reproduction-template

To Reproduce

1. Add a style rule containing an escaped space character to the CSS, e.g.:

   [data-attr=\ ] {
     color: red;
   }
   

2. pnpm build

Current vs. Expected behavior

Expect build to pass but instead throws with:

HookWebpackError: Cannot read properties of undefined (reading '0')
    at makeWebpackError (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:29:315788)
    at /project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:29:106487
    at eval (eval at create (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/webpack/bundle5.js:14:9224), <anonymous>:44:1)
-- inner error --
TypeError: Cannot read properties of undefined (reading '0')
    at /project/workspace/static/css/b0041ae41d46d821.css:1:1
    at Parser.attribute (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84899)
    at Parser.parse (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:99055)
    at Parser.loop (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:98727)
    at new Parser (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84494)
    at Processor._root (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:101743)
    at Processor._runSync (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:102250)
    at Processor.processSync (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:103005)
    at generateUniqueSelector (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:142312)
    at /project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:142631
    at /project/workspace/node_modules/.pnpm/postcss@8.4.31/node_modules/postcss/lib/container.js:369:18
caused by plugins in Compilation.hooks.processAssets
TypeError: Cannot read properties of undefined (reading '0')
    at /project/workspace/static/css/b0041ae41d46d821.css:1:1
    at Parser.attribute (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84899)
    at Parser.parse (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:99055)
    at Parser.loop (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:98727)
    at new Parser (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:84494)
    at Processor._root (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:101743)
    at Processor._runSync (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:102250)
    at Processor.processSync (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:103005)
    at generateUniqueSelector (/project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:142312)
    at /project/workspace/node_modules/.pnpm/next@15.2.0-canary.66_react-dom@19.0.0_react@19.0.0/node_modules/next/dist/compiled/cssnano-simple/index.js:320:142631
    at /project/workspace/node_modules/.pnpm/postcss@8.4.31/node_modules/postcss/lib/container.js:369:18

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4242
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.2.0-canary.66 // Latest available version is detected (15.2.0-canary.66).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Webpack

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response

Start with the linked reproduction template and run pnpm build using the CSS selector containing an escaped space. Read the stack trace around cssnano-simple's Parser.attribute and generateUniqueSelector. Done means next build completes successfully while preserving support for the escaped-space selector.