Bug: @typescript-eslint/no-redundant-type-constituents issue within declare module statement

[ernestostifano]

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.
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Issue Description

./src/modules.d.ts

declare module 'my-module' {
    import {EMyEnum} from './common.types.js';

    // INCORRECT: ESLINT HIGHLIGHTS AN ERROR
    function externalFunction(arg: EMyEnum | null): void;

    export {externalFunction};
}

ESLint: 'EMyEnum' is an 'error' type that acts as 'any' and overrides all other types in this union type. (@typescript-eslint/no-redundant-type-constituents)

It also seems to be an issue in TS:

./src/index.ts

import {externalFunction} from 'my-module';
import {internalFunction} from './utilities.js';

// INCORRECT: TS DOES NOT HIGHLIGHT THE ERROR
externalFunction('TEST');

// CORRECT: TS HIGHLIGHTS ERROR BECAUSE THE ARGUMENT MUST BE AN ENUM
internalFunction('TEST');

./src/utilities.ts

import {EMyEnum} from './common.types.js';

function internalFunction(arg: EMyEnum | null): void {
    console.log(arg);
}

export {internalFunction};

./src/common.types.ts

enum EMyEnum {
    Value = 'value'
}

export {EMyEnum};

Reproduction Repository Link

https://github.com/ernestostifano/typescript-eslint-issue-1

Repro Steps

1. Clone the repo.
2. See README.md.

Versions

package	version
@typescript-eslint/eslint-plugin	8.32.1
TypeScript	5.8.3
ESLint	9.27.0
node	22.6.0

[bradzacher]

I can't reproduce this in our playground
https://typescript-eslint.io/play/#ts=5.8.2&fileType=.tsx&code=CYUwxgNghgTiAEBbA9sArhBByRBPAtCuplvAN4BQ88AlogA7IwAu58Awsg8gHYg%2BsAvvABmMLvCxwoYZlgDcFKqLQ9ZNXvBAAPZiBg8oEAGKr1vABSwA5gC4OXRnwHwAPvB4YIASnsA3ZBpgRWUdRhY2HT0DI1M1Zg0eeEFFQQogA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1ukQBNYmvAIYtaRUrTIcmyfJXyxELVBkiJo0DtEjgwAXxAGgA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

So this indicates that it's a problem with your setup - for whatever reason we can't find the types for your common.types.js file.

Which aligns with the error message you're seeing - it mentions the type is an "error" type because TS cannot resolve the types because it cannot find the file.

[ernestostifano]

@bradzacher thanks for your feedback. You can't replicate it because you are using a non-relative import within the declaration statement and the issue happens when doing a relative import (you cannot have multi-file environments in the playground).

But then, relative imports within module declaration scope are actually not allowed and that error was being suppressed by having skipLibCheck on.

Here you can find additional details: microsoft/TypeScript#61744

I think we can close this issue.

Thanks!