Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.26.1

Plugin version

No response

Node.js version

18.17.1

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

14.3.1

Description

Hello! In my project I have conditionals for the FastifyRequest type based on the route config in FastifyContextConfig. For example, I can add a needsUserCountry config to FastifyContextConfig like this:

declare module 'fastify' {
  interface FastifyRequest<ContextConfig extends FastifyContextConfig> {
    country: ContextConfig['needsUserCountry'] extends true ? string : never;
  }

  interface FastifyContextConfig {
    needsUserCountry?: boolean;
  }
}

And then create a route like this:

app.get(
  '/countryStringLength',
  {
    config: {
      needsUserCountry: true,
    },
  },
  async (req) => req.country.length,
);

The config will be handled by a hook, so the value will be set and the type is correct.

If I set needsUserCountry to false or I don't set it, it will throw an error due to req.country being never:

And if I set it to true, it will work.

In newer releases this behavior is broken.

In 4.19.0 and 4.19.1 the config type requires url and method, passing random values makes it work. The behavior of needsUserCountry works as expected.

Since 4.20 url and method are no longer required, but now configs are not being detected, as if I didn't pass a config object.

Steps to Reproduce

Try this code with Fastify 4.18 and 4.26.1. In 4.18 it will work, in 4.26.1 types will fail.

import fastify from 'fastify';

const app = fastify();

declare module 'fastify' {
  interface FastifyRequest<ContextConfig extends FastifyContextConfig> {
    country: ContextConfig['needsUserCountry'] extends true ? string : never;
  }

  interface FastifyContextConfig {
    needsUserCountry?: boolean;
  }
}

app.get(
  '/countryStringLength',
  {
    config: {
      needsUserCountry: true,
    },
  },
  async (req) => req.country.length,
);

Expected Behavior

I expect the req object passed to the handler to have the correct types based on the config object of the endpoint.