In file included from main.cpp:1:
In file included from ./A.hpp:2:
In file included from /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/memory:80:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/unique_ptr.h:76:16: error: invalid application of 'sizeof' to an incomplete type 'A::A_impl'
        static_assert(sizeof(_Tp)>0,
                      ^~~~~~~~~~~
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0/../../../../include/c++/7.4.0/bits/unique_ptr.h:268:4: note: in instantiation of member function 'std::default_delete<A::A_impl>::operator()' requested here
          get_deleter()(__ptr);
          ^
./A.hpp:4:7: note: in instantiation of member function 'std::unique_ptr<A::A_impl, std::default_delete<A::A_impl> >::~unique_ptr' requested here
class A {
      ^
./A.hpp:10:12: note: forward declaration of 'A::A_impl'
    struct A_impl; // just forward declared
           ^
1 error generated.