Compile fails for data objects that has copy constructor deleted

[zhqrbitee]

If the data object has its copy constructor deleted, e.g.

class Data {
public:
   Data(....) {...};

   Data(const Data&) = delete;
   Data(Data&&) { ...; }
   Data &operator=(const Data &other) = delete;
   Data &operator=(Data &&other) noexcept { ...; }
}

Compilation will fail:

emhash/hash_table6.hpp:1438:41: note: in instantiation of member function 'emhash6::...::operator=' requested here
            if (is_copy_trivially())
                EMH_PKV(_pairs, bucket) = EMH_PKV(_pairs, next_bucket);    <---
            else
                EMH_PKV(_pairs, bucket).swap(EMH_PKV(_pairs, next_bucket));

Compiler will try to compile both branches so it fails.
If we are in C++17, a easy fixes would be mark the branch with constexpr, so the compiler won't compile the first branch.

[ktprime]

emhash is flat hash map which must support c++11/14, and key/value must be swappable or copy/move constructible

it will be changed to
EMH_PKV(_pairs, bucket) = std::move(EMH_PKV(_pairs, next_bucket));