Due to GCC 4.7.2 and lower not supporting a change in template deduction rules as explained in the answer below

http://stackoverflow.com/questions/14808663/stdunordered-mapemplace-issue-with-private-deleted-copy-constructor/14836911#14836911

the following code will not compile:

#include <utility>

class test

{

public:

test() =default;

private:

test(test const&);

};

int main()

{

std::is_convertible<test&, test>::value;

};

For the same reason, it is not possible to use classes which use the BOOST_MOVABLE_BUT_NOT_COPYABLE in situations where libstdc++ uses std::is_convertible, for instance when using piecewise construction in std::unordered_map::emplace() (I came across the issue when making an unordered_map of boost::file_mapping)

As deleting the copy constructor fixes the issue (i.e. test(test const&) =delete;), would it be at all possible for BOOST_MOVABLE_BUT_NOT_COPYABLE to do the same for compilers that support the syntax?