golang.org/x/sync/errgroup returns the first non-nil error (if any) of all go routines managed by it. With the introduction of wrapped errors in go 1.20 this package could be updated to return a single error that wraps all errors returned by the routines started with it. The change would be minimal, essentially only replacing the errOncer with a mutex and errors.Join.

I am unsure if the change would breaking existing code using errgroup. At the moment users shouldn't rely on any specify error be returned, while with the change one can check if a specific error has occurred with errors.Is or errors.As.