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• I have tried restarting my IDE and the issue persists.
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Relevant Package

parser

Playground Link

No response

Repro Code

N/A. This issue is about a confusing configuration behavior.

ESLint Config

// base-config.js
module.exports = {
  plugins: [ "@typescript-eslint" ],
  parser: "@typescript-eslint/parser",
  parserOptions: { projectService: true }
}

// .eslintrc.js
module.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 both projectService and project are set. This can be particularly confusing when extending a configuration that sets parserOptions 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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