Add a specific error message for disallowed imports in project references?

Suggestion

🔍 Search Terms

project reference back import

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

After introducing project references to a monolith repo with many developers, I've sometimes seen confusion over people writing imports from projects in the incorrect direction: e.g. if project B has a tsconfig reference to project A, trying to import from B in A. ♻️

https://github.com/JoshuaKGoldberg/ts-repro-back-reference-import

Here's an example error message that says what's technically wrong but doesn't give much context:

src/a/index.ts:1:19 - error TS6307: File '---/src/b/index.ts' is not listed within the file list of project '---/src/a/tsconfig.json'. Projects must list all files or use an 'include' pattern.

1 import { b } from "../b";
                    ~~~~~~

📃 Motivating Example

Adding a specific error for these incorrect direction ("back") imports might help folks new to project references understand what they're doing wrong:

src/a/index.ts:1:19 - error TS6307: File '---/src/b/index.ts' is not listed within the file list of project '---/src/a/tsconfig.json'. Projects must list all files or use an 'include' pattern.

  File "---/b/index.ts" is included in "---/b/tsconfig.json", but "---/a/tsconfig.json" does not list a reference to "---/b/tsconfig.json".

1 import { b } from "../b";
                    ~~~~~~

💻 Use Cases

https://github.com/JoshuaKGoldberg/ts-repro-back-reference-import

Start by reproducing the back-reference import in the linked ts-repro-back-reference-import repository and inspect the involved tsconfig.json project references. Compare the existing TS6307 diagnostic with the proposed additional context, then identify the TypeScript compiler diagnostic and test locations that handle project-file inclusion. Done means the incorrect import produces a specific, actionable message without changing valid project-reference behavior.