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template <class Result, class... Params>
class Slot
{
public:

    virtual Result operator()(Params ... args) = 0;
};

using IntSlot = awl::Slot<void, int>;

class SlotA : public IntSlot {};
class SlotB : public IntSlot {};

class Handler :
    public SlotA,
    public SlotB
{
public:

    void SlotA::operator()(int value) override
    {
        lastValueA = value;
    }

    void SlotB::operator()(int value) override
    {
        lastValueB = value;
    }

    int lastValueA = 0;
    int lastValueB = 0;
};

error C2535: 'void `anonymous-namespace'::Handler::operator ()(int)': member function already defined or declared

struct SlotA
{
    virtual void func(int value) = 0;
};

struct SlotB
{
    virtual void func(int value) = 0;
};


class MultiObserver :
    public SlotA,
    public SlotB
{
public:

    void SlotA::func(int value) override
    {
        aValue = value;
    }
    
    void SlotB::func(int value) override
    {
        aValue = value;
    }

    int aValue = 0;
    int bValue = 0;
};

void f()
{
    MultiObserver mo;
}

error: cannot define member function ‘{anonymous}::SlotA::func’ within ‘{anonymous}::MultiObserver’
error: cannot define member function ‘{anonymous}::SlotB::func’ within ‘{anonymous}::MultiObserver’