This has now created another issue where we cannot use a FastifyBaseLogger instance where a pino's BaseLogger instance is expected. Because now FastifyBaseLogger deviated from BaseLogger by not adding the newest get msgPrefix() property. Was it intended to change it this way? Or at least add this breaking change in a non major version?

const app = fastify()

something(app.log)
//            ^
// Argument of type 'FastifyBaseLogger' is not assignable to parameter of type 'BaseLogger'.

function something(logger: BaseLogger) {
  logger.info('Hello, world!')
}

From my understandings, this change comes from #6286 where typescript correctly warned that fastify CI was missing a field to match the new structure of BaseLogger.

Ihmo, this PR should be reverted and #6286 should be merged instead.

cc @jsumners @gurgunday @dancastillo

Fastify uses a generic logger interface that requires a limited set of methods:

• https://fastify.dev/docs/latest/Reference/Server/#loggerinstance

This PR fixes this issue: we don't want to mock the pino interface, we want to ship pino included.
For this reason, I would not revert it, but I'm open to the @fastify/typescript decision.

As general fix, I suggest to don't use module's logger in the code

type MyApplicationLogger = FastifyBaseLogger  as BaseLogger

This simplify these kind of changes.

Or at least add this breaking change in a non major version?

ATM we follow the principale like the typescript module does: types may break in minor changes.

@Eomm BaseLogger doesn't have .child(), thus using BaseLogger won't work in many use cases.