Environment information

CLI:
  Version:                      2.3.6
  Color support:                true

Platform:
  CPU Architecture:             aarch64
  OS:                           macos

Environment:
  BIOME_LOG_PATH:               unset
  BIOME_LOG_PREFIX_NAME:        unset
  BIOME_CONFIG_PATH:            unset
  BIOME_THREADS:                unset
  NO_COLOR:                     unset
  TERM:                         xterm-256color
  JS_RUNTIME_VERSION:           v22.21.1
  JS_RUNTIME_NAME:              node
  NODE_PACKAGE_MANAGER:         pnpm/9.13.2

Biome Configuration:
  Status:                       Loaded successfully
  Formatter enabled:            true
  Linter enabled:               true

Rule name

nursery/useConsistentArrowReturn

Playground link

https://biomejs.dev/playground/?lintRules=useConsistentArrowReturn&files.main.tsx=YwBvAG4AcwB0ACAAZwBlAHQAUwBjAGgAZQBtAGEAUgBvAHcAVAB5AHAAZQBzACAAPQAgACgAbAA6ACAAcwB0AHIAaQBuAGcAKQAgAD0APgAKACAAIABsAAoAIAAgACAAIAAuAHMAcABsAGkAdAAoACIAXABuACIAKQA%3D&files.biome.json=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%3D%3D

Expected result

Current Behavior (Bug)

When running biome lint --fix via CLI with the useConsistentArrowReturn rule configured with style: "always", the autofix produces semantically incorrect code that changes the function's behavior.

Input code:

const foo = (l: string) =>
  l
    .split('\n')

CLI autofix output (INCORRECT):

const foo = (l: string) =>
  {
  return
  l
    .split("\n");
}

Problem: The function now returns undefined instead of the split array because there's a newline after return, triggering JavaScript's automatic semicolon insertion (ASI). This silently breaks working code by changing its runtime behavior.

Expected Behavior

The autofix should produce:

const foo = (l: string) => {
  return l.split('\n');
};

This is what VS Code's Quick Fix (source.fixAll.biome) correctly produces when applying the same lint rule fix.

Configuration Used

{
  "linter": {
    "rules": {
      "nursery": {
        "useConsistentArrowReturn": {
          "level": "warn",
          "options": { "style": "always" }
        }
      }
    }
  }
}

Steps to Reproduce

1. Save the input code to a file (e.g., test.ts)
2. Run: biome lint --fix test.ts
3. Observe the incorrect output with return on a separate line from the value

Impact

This bug can silently break working code by changing its runtime behavior:

• The function returns undefined instead of the intended value
• The code remains syntactically valid, so the error may not be caught until runtime
• This is a critical semantic change that could cause production bugs

Additional Context

• VS Code's Quick Fix works correctly, suggesting the issue is specific to the CLI autofix implementation
• According to Discord discussion, VS Code likely runs format after applying the fix, which corrects the incorrect return placement
• The CLI autofix should produce correct code without requiring a subsequent format pass
• This was discussed in the Biome Discord where maintainer Siketyan confirmed this is a bug