@jonasschnelli wrote in #14979:
> #14573 broke < Qt5.6 compatibility due to calling the lambda version of `addAction` that was added in Qt5.6.

We currently [support](https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md) QT >=5.2, but hardly anyone tests with that. If we make one of the Travis machines use it, we should have a better change of catching this.

See also discussion about minimum QT version in #13478 
#include <future>
#include <utility>
#include <boost/signals2/signal.hpp>
struct ValidationInterfaceConnections {
    boost::signals2::scoped_connection UpdatedBlockTip;
    boost::signals2::scoped_connection TransactionAddedToMempool;
    boost::signals2::scoped_connection BlockConnected;
    boost::signals2::scoped_connection BlockDisconnected;
    boost::signals2::scoped_connection TransactionRemovedFromMempool;
    boost::signals2::scoped_connection ChainStateFlushed;
    boost::signals2::scoped_connection Broadcast;
    boost::signals2::scoped_connection BlockChecked;
    boost::signals2::scoped_connection NewPoWValidBlock;
};
struct MainSignalsInstance {
    boost::signals2::signal<void (const CBlockIndex *, const CBlockIndex *, bool fInitialDownload)> UpdatedBlockTip;
    boost::signals2::signal<void (const CTransactionRef &)> TransactionAddedToMempool;
    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex, const std::vector<CTransactionRef>&)> BlockConnected;
    boost::signals2::signal<void (const std::shared_ptr<const CBlock> &)> BlockDisconnected;
    boost::signals2::signal<void (const CTransactionRef &)> TransactionRemovedFromMempool;
    boost::signals2::signal<void (const CBlockLocator &)> ChainStateFlushed;
    boost::signals2::signal<void (int64_t nBestBlockTime, CConnman* connman)> Broadcast;
    boost::signals2::signal<void (const CBlock&, const CValidationState&)> BlockChecked;
    boost::signals2::signal<void (const CBlockIndex *, const std::shared_ptr<const CBlock>&)> NewPoWValidBlock;
    
    
    
    SingleThreadedSchedulerClient m_schedulerClient;
    std::unordered_map<CValidationInterface*, ValidationInterfaceConnections> m_connMainSignals;
    explicit MainSignalsInstance(CScheduler *pscheduler) : m_schedulerClient(pscheduler) {}