declaration merging is the technique we use in Fastify to decorate the types of our base objects. See https://github.com/fastify/fastify-static/blob/d40ce2d33481da7abcd261b1e8699ef31a9e2854/types/index.d.ts#L8-L17.

This technique has a major flaw: once a module is "seen" by TypeScript, those types are mergerd in original definition, even if the decorating plugin is not loaded in the current plugin tree.

After studying a bit of TypeScript, I think we can do better. In essence, we should be able to support something like:

import fp from 'fastify-plugin'
import { FastifyStaticMixin } from '@fastify/static'

export default fp(async function (app: FastifyInstance & FastifyStaticMixin, opts: object) {

})

Or even better:

import fp from 'fastify-plugin'
import { FastifyStaticMixin } from '@fastify/static'

export default fp(async function (app: FastifyInstance & FastifyStaticMixin, opts: object) {

}, {
  decorators: {
    reply: FastifyStaticMixin
  }
})

Or some other analog syntax with native Fastify, e.g. withMixin<T>().

I think this would be a significant improvement on our TS story for v5. Wdyt?