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• I have tried restarting my IDE and the issue persists.
• I have updated to the latest version of the packages.
• I havesearched for related issues and found none that matched my issue.
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Relevant Package

parser

Playground Link

No response

Repro Code

N/A.Thisissue isaboutaconfusingconfigurationbehavior.

ESLint Config

// base-config.jsmodule.exports={plugins:["@typescript-eslint"],parser:"@typescript-eslint/parser",parserOptions:{projectService:true}}// .eslintrc.jsmodule.exports={extends:"./base-config.js",parserOptions:{project:"./tsconfig.eslint.json"}}

tsconfig

{"compilerOptions": {// ...  }}

Expected Result

The docs aren't clear of what the parser will use when bothprojectService andproject are set. This can be particularly confusing when extending a configuration that setsparserOptions which are later combined into a single merged object. I would expect for the parser to throw an explicit error in such misconfiguration scenarios, or have a well-documented explanation of what happens when both settings are provided.

Actual Result

eslint --print-config shows that the merged configuration has{ project: "./tsconfig.eslint.json", projectService: true }, and it's not clear how the TypeScript program will be discovered.

Additional Info

No response

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