Related Issues

• proposal: errors: add Errors as a standard way to represent multiple errors as a single error #47811 (closed)
• proposal: errors: add Errors function to undo Join #57358 (closed)
• proposal: errors: unwarap joinError to avoid deeply-nested joins #65676 (closed)
• proposal: errors: add NewJoinedErrors #48831 (closed)
• errors: add support for wrapping multiple errors #53435 (closed)
• errors: Using `errors.Join` on a `joinError` should expand the existing joined error #60209 (closed)
• proposal: define a type for (type, error) #64377 (closed)
• errors: errors.Join does not skip nil error interface implementations #70626 (closed)
• proposal: composite errors #20984 (closed)

Related Code Changes

• errors: unwrap joinError to avoid deeply-nested joins

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#47811 looks like a similar proposal, but I feel like this proposal is much smaller in size/scope.

It is not possible to change the return type like this because it is an observable breaking change.
Consider this code:

err := errors.Join(errs...)
err = fmt.Errorf("some context %s: %w", str, err)

it'll fail to build if we change the return type because the inferred type for err wont be compatible with fmt.Errorf's return type.

@Jorropo I am sorry, but I don't entirely follow. Are you saying that making errors.joinError public and adding it to errors.Join return type would somehow break fmt.Errorf(...) backwards compability? errors.JoinError would implement func Unwrap() []error just like before, and according to https://pkg.go.dev/fmt#Errorf, the %w verb will not rely on the type, but look at the Unwrap method. I might of course be missing something here.

The only critical problem is changing the return type of errors.Join.

The problem I am demonstrating is not with fmt.Errorf it is with implicit type conversions of the = operator.
Here is more minimal code if you want:

err := errors.Join(nil)
var errr error
err = errr

this would compile before your change but not after.

Tl;Dr

you can't change the signature of errors.Join, random changes you could do to the proposal:

• remove that part and rely on people manually instantiating errors.MultipleErrors
• deprecate errors.Join and make an errors.Join2 (better name pending) function with the updated signature
• ...

closing as not feasible