Before You File a Bug Report Please Confirm You Have Done The Following...

• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I havesearched for related issues and found none that matched my issue.
• I haveread the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=5.9.3&fileType=.tsx&code=CYUwxgNghgTiAEYD2A7AzgF3lAXPA3vAEYD8emMAligObwC%2BA3AFDOUBm8AFAIRTwAfAdgB0ReAF4p8AETskSGQEoCzePGb0gA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Y6RAM0WloHsalfkwCG8WmQAWo5uigB3UdCaRwYAL4gNQA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

Repro Code

declareconsta:{b?:string};if(!a||a.b==="foo"){}

ESLint Config

module.exports={parser:"@typescript-eslint/parser",rules:{"@typescript-eslint/prefer-optional-chain":["error",{}],},};

tsconfig

{"compilerOptions": {}}

Expected Result

I expect that no error from@typescript-eslint/prefer-optional-chain is triggered on the third line, as an optional chain would change behavior.

Actual Result

An error from@typescript-eslint/prefer-optional-chain is triggered on the third line. When autofixed, this produces the code

declareconsta:{b?:string};if(a?.b==="foo"){}

Whena.b is undefined, the original comparison produced the valuetrue, executing the code within theif-block. The 'autofixed' code producesfalse in that case, skipping the code within theif-block.

Additional Info

This seems due to the changes in#11533. The comment in#11533 (comment) already hints at this incorrect behavior.