C++ can not find method (boost signals2)

Question

here is code, c++11:

#include <string>
#include <boost/signals2/signal.hpp>

namespace messages {
    struct Event1 final {};
    struct Event2 final {};

    template <typename T, typename Tag>
    class EventHandler {
    protected:
        bool cache_valid_ = false;
        T cache_val_;
        boost::signals2::signal<void (const T&)> signal_;
        using slot_type = typename decltype(signal_)::slot_type;
        using connection = boost::signals2::connection;
    public:
        EventHandler() {}
        void emit(Tag, const T& val) {
            if (val != cache_val_) {
                cache_valid_ = true;
                cache_val_ = val;
                signal_(val);
            }
        }

        connection subscribe(Tag, slot_type slot) {
            if (cache_valid_)
                slot(cache_val_);
            return signal_.connect(slot);
        }
    };
}

struct MessageBus : public messages::EventHandler<int, messages::Event1>,
            public messages::EventHandler<std::string, messages::Event2> {
};

int main()
{
    MessageBus bus;
    bus.subscribe(messages::Event1(), [](const int& val) {});
}

recent versions of gcc and clang can not compile this code with -std=c++11, they can not decide what overload to choose here: bus.subscribe(messages::Event1(), [](const int& val) {});.

1) Why, they can not choose appropriate method? There is only one MessageBus::subscribe, which accept Event1 type as first argument

2)How fix this issue, without MessageBus modifications?

I can fix this issue by this code:

struct MessageBus : public messages::EventHandler<int, messages::Event1>,
            public messages::EventHandler<std::string, messages::Event2> {
    using messages::EventHandler<int, messages::Event1>::subscribe;
    using messages::EventHandler<std::string, messages::Event2>::subscribe;
};

but the whole idea behind messages::EventHandler to reduce efforts for introduction of new message, so it would be good to fix this problem by modification messages::EventHandler in some way.