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Bug: [no-unnecessary-type-assertion] false positive when casting changes the inferred type from a generic #5359

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DetachHead opened this issue Jul 21, 2022 · 3 comments
Closed
4 tasks done

Bug: [no-unnecessary-type-assertion] false positive when casting changes the inferred type from a generic #5359

DetachHead opened this issue Jul 21, 2022 · 3 comments
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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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@DetachHead
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DetachHead commented Jul 21, 2022 •
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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 have searched for related issues and found none that matched my issue.
  • I have read the FAQ and my problem is not listed.

Playground Link

https://typescript-eslint.io/play/#ts=4.7.2&sourceType=module&code=CYUwxgNghgTiAEYD2A7AzgF3gMyUgXPADwAqANPAKoB8AFAG5QQCuIhlAlPALzXwkBYAFDCA9KIDmIFCBgBLMGniwEKJAHd4clNllxgypURTMAtgCNZFTPJQS6jFm3g3tErr3gmLs4cnRY5rA8OHi0AEThXFBK3pYw8OJ6SDCEagC0zCgyYCBoaLAAnukYhQAOIOkxaLIYcqhAA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0ipWkOTJE0fJQ5N0UOdA7RI4MAF8QOoA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA

Repro Code

declare const foo: <T, U>(value: U) => T

//generics are now inferred as <number, string>(value: string) => number
const bar = foo("") as number //error: no-unnecessary-type-assertion

ESLint Config

module.exports = {
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
};

tsconfig

{
  "compilerOptions": {
    "strictNullChecks": true
  }
}

Expected Result

no error

Actual Result

error: no-unnecessary-type-assertion

Additional Info

sometimes it's useful to use a cast instead of specifying the generic, in case you only want to have to specify one type while still having the other types inferred.

Versions

package version
@typescript-eslint/eslint-plugin 5.30.7
@typescript-eslint/parser 5.30.7
TypeScript 4.7.2
ESLint 8.15.0
@DetachHead DetachHead added bug Something isn't working package: eslint-plugin Issues related to @typescript-eslint/eslint-plugin triage Waiting for team members to take a look labels Jul 21, 2022
@Josh-Cena
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Josh-Cena commented Jul 21, 2022 •
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Try document.querySelectorAll<HTMLImageElement>("")? Assertions are inference sites so they impact the generic function's type.

@DetachHead DetachHead changed the title Bug: [no-unnecessary-type-assertion] false positive when casting NodeListOf Bug: [no-unnecessary-type-assertion] false positive when casting changes the inferred type from a generic Jul 21, 2022
@DetachHead
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that worked, thanks! interesting i had no idea casting effected generic inference.

in that case i've updated the issue with a better example

@bradzacher
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bradzacher commented Jul 21, 2022 •
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Yup, Josh is right.

When a generic Is only used in the return type then TS will propagate the asserting into the generic.

Using a generic in a return type is generally a code smell and breaks the golden rule of generics (a generic in a function should be used at least twice) https://effectivetypescript.com/2020/08/12/generics-golden-rule/

Check out thus example which shows how TS "backfills" the generic based on the assertion

declare function foo<T>(): T;

foo<string>();
foo() as string;

playground

@bradzacher bradzacher closed this as not planned Won't fix, can't repro, duplicate, stale Jul 21, 2022
@bradzacher bradzacher added working as intended Issues that are closed as they are working as intended and removed bug Something isn't working triage Waiting for team members to take a look labels Jul 21, 2022
@github-actions github-actions bot locked as resolved and limited conversation to collaborators Aug 21, 2022
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