[prefer-optional-chain] fixer produces wrong logic

[bradleyayers]

Repro

{
  "rules": {
    "@typescript-eslint/prefer-optional-chain": ["error"]
  }
}

declare const a: { b: string | null } | null;
a !== null && a.b !== null && a.b.length > 0;
/*
^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer using an optional chain expression instead, as it's more concise and easier to read.
*/

Expected Result

a?.b != null && a.b.length > 0;

Actual Result

a?.b !== null && a.b.length > 0;
/*
                 ^ Object is possibly 'null'.
*/

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	2.15.0
@typescript-eslint/parser	2.15.0
TypeScript	3.7.3
ESLint	6.1.0
node	10.16.0
npm	6.9.0

[bradzacher]

interesting.
Types wise, this doesn't work because of the !==, ?. produces an undefined. If you instead use != null, then it's fine.

The root cause of this is due to a limitation in the logic that I have been wanting (needing) to fix in that it only allows !=/!== binary expressions.

[glen-84]

I have a similar issue, but I use strict eqeqeq, so I wouldn't want the fix to use !=.

if (coreStore.activeSiteSection && coreStore.activeSiteSection.backgroundImageUrl !== null) {}

Wants to change to:

if (coreStore.activeSiteSection?.backgroundImageUrl !== null) {}

... but this has different behaviour, because undefined !== null, so it would enter the block. It shouldn't flag this case.