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=5.7.3&fileType=.ts&code=GYVwdgxgLglg9mABFApgZygCgIYH4BciARnHADYrZgCUhJ5lSA3gLABQiiATilCF0myIAhAF5RiYNjJoUAbnYBfdu1QZM1OUA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0ARhw5IhTWo3yJoQ%2BLTIcAtsRGJ0UJdA7RI4MAF8QZoA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eFYAZgIaonrhgC%2BIfkA&tokens=false

Repro Code

functiontest(a?:boolean):boolean{returna!==false;}test();

ESLint Config

{"rules":{"@typescript-eslint/no-unnecessary-boolean-literal-compare":"error"}}

tsconfig

{"compilerOptions": {"strict":false  }}

Actual Result

With the configuration above (withoutstrictNullChecks), the@typescript-eslint/no-unnecessary-boolean-literal-compare rule emits a (false) error. Applying the automatic fix translates

functiontest(a?:boolean):boolean{returna!==false;}

into

functiontest(a?:boolean):boolean{returna;}

which is obviously not equivalent asa is nullable.

Root cause seems to be that withoutstrictNullChecks, TypeScript reports the type ofa asboolean instead ofboolean | undefined, making typescript-eslint think thata is not nullable ⇒a !== false is equivalent to justa.

My guess is that theno-unnecessary-boolean-literal-compare rule absolutely has to check forstrictNullChecks and refuse to work (like other rules do, such asprefer-nullish-coalescing).

We already broke our app in runtime after applyingeslint --fix without reviewing each and every change. We can't enablestrict /strictNullChecks as it would emit thousands of errors in the legacy codebase.

Expected Result

Either of
a. (best option) no error, like withstrictNullChecks: true (I assume, not technically possible)
b. eslint refuses to work if theno-unnecessary-boolean-literal-compare rule is enabled, butstrictNullChecks aren't