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• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

Relevant Package

parser

My proposal is suitable for this project

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

I've heard of two requests around EXPERIMENTAL_useProjectService:

• Customize the default options in the case of a tsconfig.json not existing: https://discord.com/channels/508357248330760243/640177429775777792/1163326359100199012
• Warn if it's being used: feat(typescript-estree): add allowDefaultProjectForFiles project service allowlist option #7752 (comment)

These are technically two requests, but I think it'd be good to discuss them together. Whatever API we end up going with may need to accommodate both.

Proposal: how about a fallbackCompilerOptions?

type FallbackCompilerOptions = 
  /* Don't allow fallback/inferred projects at all */
  | "error" 
  /* Allow fallback/inferred projects but log a warning */
  | "warn"
  /* Directly use these */
  ts.CompilerOptions;

// (approximation of the shape we'd allow)
interface ProjectServiceParserOptions {
  EXPERIMENTAL_useProjectService: true;

  // If undefined, go with TypeScript's defaults
  fallbackCompilerOptions?: FallbackCompilerOptions;
}

Additional Info

Courtesy of @jakebailey in https://discord.com/channels/508357248330760243/1163326359100199012/1163526441103802398: project services have a setCompilerOptionsForInferredProjects we can use.

#7752 largely enables the case of an inferred project in the first place.

Edit: oh, and also mentioned here: #6575 (comment)