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[no-unnecessary-type-assertion] False positive with lazily initialized local variable #2423

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fasttime opened this issue Aug 25, 2020 · 3 comments
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[no-unnecessary-type-assertion] False positive with lazily initialized local variable #2423

fasttime opened this issue Aug 25, 2020 · 3 comments
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fix: strictNullChecks issues that were fixed by turning on strictNullChecks package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin working as intended Issues that are closed as they are working as intended

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@fasttime
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fasttime commented Aug 25, 2020 •
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Not sure if or how this relates to #2248. In my case, the repro is quite simple.

Repro

.eslintrc.json

{
    "parser": "@typescript-eslint/parser",
    "parserOptions": {
        "project": "./tsconfig.json"
    },
    "plugins": [
        "@typescript-eslint"
    ],
    "rules": {
        "@typescript-eslint/no-unnecessary-type-assertion": [
            "error"
        ]
    }
}

index.ts

let foo: string[];
Reflect.apply(() => foo = ['bar'], null, []);
console.log(foo![0]);

package.json

{
  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "latest",
    "@typescript-eslint/parser": "latest",
    "eslint": "7.4.0",
    "typescript": "4.0.2"
  }
}

tsconfig.json

{
    "compilerOptions": {
        "strictNullChecks": false
    }
}

Run with: npx eslint index.ts

Expected Result

No error.

Actual Result

Getting error This assertion is unnecessary since it does not change the type of the expression @typescript-eslint/no-unnecessary-type-assertion on line 3.

Versions

package version
@typescript-eslint/eslint-plugin 3.10.0
@typescript-eslint/parser 3.10.0
TypeScript 4.0.2
ESLint 7.4.0
node 14.8.0
@fasttime fasttime added package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin triage Waiting for team members to take a look labels Aug 25, 2020
@bradzacher
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I am unable to repro this against master.
Are you using strictNullChecks?

@bradzacher bradzacher added awaiting response Issues waiting for a reply from the OP or another party unable to repro issues that a maintainer was not able to reproduce and removed triage Waiting for team members to take a look labels Aug 31, 2020
@fasttime
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I am not using strictNullChecks, and I am still getting the error after updating @typescript-eslint/eslint-plugin and @typescript-eslint/parser to version 4.0.1 (I didn't try cloning the repo and building the packages from master).
I updated the repro with complete versions of the relevant files, that should help reproduce the issue in case the previous repro was missing information.

@bradzacher bradzacher added fix: strictNullChecks issues that were fixed by turning on strictNullChecks working as intended Issues that are closed as they are working as intended and removed awaiting response Issues waiting for a reply from the OP or another party unable to repro issues that a maintainer was not able to reproduce labels Aug 31, 2020
@bradzacher
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bradzacher commented Aug 31, 2020 •
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Unfortunately the rule cannot correctly detect this case with strictNullChecks turned off.
This is by-design in TS itself - TS does not do use before assigned analysis on code without this compiler option. Because it doesn't do this analysis, our rule has no information to work from.

Note that with the option off, you can actually omit the non-null assertion with strictNullChecks, and the code is still compile-time safe and will not throw any TS errors.

You should turn the option on, as having it off invites a ridiculous amount of unsafety into your codebase.

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fix: strictNullChecks issues that were fixed by turning on strictNullChecks package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin working as intended Issues that are closed as they are working as intended
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