AccountManager.swift : Unable to infer closure type in the current context

[shivanraptor]

I'm compiling the source and the error "Unable to infer closure type in the current context" pops up:

public func temporaryAccount(manager: AccountManager, rootPath: String) -> Signal<TemporaryAccount, NoError> {
    return manager.allocatedTemporaryAccountId()
    |> mapToSignal { id -> Signal<TemporaryAccount, NoError> in
        let path = "\(rootPath)/\(accountRecordIdPathName(id))"
        return openPostbox(basePath: path + "/postbox", globalMessageIdsNamespace: Namespaces.Message.Cloud, seedConfiguration: telegramPostboxSeedConfiguration)
        |> mapToSignal { result -> Signal<TemporaryAccount, NoError> in
            switch result {
                case .upgrading:
                    return .complete()
                case let .postbox(postbox):
                    return .single(TemporaryAccount(id: id, basePath: path, postbox: postbox))
            }
        }
    }
}

The error occurs on mapToSignal line (line 3 above). How could I fix this?