How to make std::make_unique a friend of my class

make_unique perfect forwards the arguments you pass to it; in your example you're passing an lvalue (x) to the function, so it'll deduce the argument type as int&. Your friend function declaration needs to be

friend std::unique_ptr<A> std::make_unique<A>(T&);

Similarly, if you were to move(x) within CreateA, the friend declaration would need to be

friend std::unique_ptr<A> std::make_unique<A>(T&&);

This will get the code to compile, but is in no way a guarantee that it'll compile on another implementation because for all you know, make_unique forwards its arguments to another internal helper function that actually instantiates your class, in which case the helper would need to be a friend.