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Issue

When this PR was merged to main in this commit, it also removed all the changes made in this PR

Ideally AFAIC PR 2981 would be re-added. This might be a bit problematic since #3203 hijacked the first generic slot for all decorate* functions, so re-adding it would be a breaking change. Also this means that since typed decorators wasn't actually added in v4, the feature isn't and cannot be used by anyone.

There's few options I can think of:

1. Re-add typed decorators as a supplement to 'this types' even if it's a (very small) breaking change
2. Add typed decorators to decorate* functions in fastify v5
3. Introduce typedDecorate* functions that do the same as decorate* but with type checking
4. Forget that typed decorators can even exist and ignore this issue

There's also this tiny documentation change that should probably be removed now: https://github.com/fastify/fastify/pull/2981/files#diff-c2db295d2a99e431bb24ac08c3953353dbb702f62b48587aa2cd76a0b78e0dbf