Change https://go.dev/cl/578735 mentions this issue: ssh: allow to dynamically change authentication methods

CC @golang/security

The idea behind the proposal is sound: having a way to switch auth methods without returning PartialSuccess.

I suggest creating an interface (internal or exported) that both PartialSuccessError and ChangeAuthMethodsError implement to use in the type assertion here.

// ChangeAuthMethods can be implemented by any error returned from auth callbacks to 
// change the set of auth callbacks used for further attempts on this connection.
type ChangeAuthMethods interface {
    NextAuthMethods() ServerAuthCallbacks
}

func (e *PartialSuccessError) NextAuthMethods() ServerAuthCallbacks { return e.Next }
func (e *ChangeAuthMethodsError) NextAuthMethods() ServerAuthCallbacks { return e.Next }

An exported interface has the benefit of using error values to affect other behaviors, like sending banner messages.

I don't think a ChangeAuthMethods interface composes well with BannerError for example, because we'd have to add NextAuthMethods to BannerError ourselves.

Instead, we already have a mechanism to nest error types, which is Unwrap.

We can document that we check for both BannerError, PartialSuccessError, and ChangeAuthMethodsError with errors.As, and then implementations can just compose them by nesting a ChangeAuthMethodsError in a BannerError (or more complex custom nesting by using their own type which Unwraps to all of those).

Good point, Unwrap is a better way to combine errors. Should we implement Unwrap for all 3 proposed error types, so they can be combined in an arbitrary order?