• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I haveread the FAQ and my problem is not listed.

Repro

{"rules": {"@typescript-eslint/non-nullable-typer-assertion-style":"error"  }}

functionfirst<T>(array:ArrayLike<T>):T|null{returnarray.length>0 ?(array[0]asT) :null;}

tsconfig:

"compilerOptions": {"target":"es5","module":"commonjs","strict":true,"noUncheckedIndexedAccess":true  },

Expected Result

typescript-eslint should not report any errors.

Actual Result

typescript-eslint complains about this type assertion:array[0] as T:

Use a ! assertion to more succintly remove null and undefined from the type @typescript-eslint/non-nullable-type-assertion-style

Additional Info

When compiling withnoUncheckedIndexedAccess, the type assertionarray[0] as T is required because the original type ofarray[0] isT | undefined.

However, non-nullable-type-assertion-style complains about the cast, suggesting that it should bearray[0]! instead. This is not correct because the type parameterT might itself be nullish.array[0]! is equivalent toarray[0] as NonNullable<T>, which is not the same asarray[0] as T.

To see the issue more clearly, consider the following slightly more contrived example:

functionfirst<Textendsstring|null>(array:ArrayLike<T>):T|null{returnarray.length>0 ?(array[0]asT) :null;}

non-nullable-type-assertion-style complains about this code too, but here the problem is more obvious. Clearly the type assertionarray[0] as T is not equivalent toarray[0]! because the type parameterT has a constraint that explicitly allows the type to benull.

See#4509 for a fix

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