return
	}

	// Special case: Is, As and Unwrap only apply when type
	// implements error.
	if id.Name == "Is" || id.Name == "As" || id.Name == "Unwrap" {
		if recv := sign.Recv(); recv == nil || !implementsError(recv.Type()) {
			return
		}
	}

	// Special case: Unwrap has two possible signatures.
	// Check for Unwrap() []error here.
	if id.Name == "Unwrap" {
		if args.Len() == 0 && results.Len() == 1 {