Prerequisites

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

Issue

Premise

Not really sure if this is a feature or a bug since methods on Reply that modify what the response schema should be, don't actually narrow at the type level when there is a broader type available.

For example:

fastify.post('/',
    {
      schema: {
        response: {
          200: Type.Literal('Success!'), // strict string literal
          500: Type.String(), // broader string
        },
      },
    } as const,
    async (_, reply) => {
        // this should be a type error but it isn't
        reply.code(200).send('Hiya!');
    }
);

The reason it does not show a type error is because the type applied on the payload is string | 'Success! and since string is broader than 'Success!', it only applies string.

Proposed Solution

The type can be narrowed by modifying generics in:

to something like this:

code<T extends keyof SchemaCompiler['response']>(statusCode: T): FastifyReply<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig, SchemaCompiler, TypeProvider, CallTypeProvider<TypeProvider, SchemaCompiler['response'][T]>>;

For consistency, this would need to be applied to the status method as well.

Furthermore, some extra Typescript-ing would be needed to account for response schemas with keys like 2xx but should be relatively simple to implement with string literals.

I would be willing to make the pull request for this if this is something that seems useful.