proposal: x/sync: change errgroup to return a wrapped error instead of only the first

[fasmat]

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.

[apparentlymart]

I raised a similar idea in the discussion of #57534 and the original author of errgroup responded that it's more appropriate to return only the error that causes the context to be cancelled.

That seems plausible to me so I didn't push that idea any further, but if you intend to pursue this proposal you may wish to consider that feedback and incorporate a response into your proposal.

[seankhliao]

Duplicate of #23595