Why construction of std::unique_ptr for uncomplete type compiles?

Code:

#include <memory>

struct Data;
std::unique_ptr<Data> make_me();

int main()
{
    std::unique_ptr<Data> m = make_me();
    return 0;
}

Which of course fails:

In file included from <source>:1:
In file included from /opt/compiler-explorer/gcc-7.1.0/lib/gcc/x86_64-linux-gnu/7.1.0/../../../../include/c++/7.1.0/memory:80:
/opt/compiler-explorer/gcc-7.1.0/include/c++/7.1.0/bits/unique_ptr.h:76:16: error: invalid application of 'sizeof' to an incomplete type 'Data'
        static_assert(sizeof(_Tp)>0,
                      ^~~~~~~~~~~
/opt/compiler-explorer/gcc-7.1.0/include/c++/7.1.0/bits/unique_ptr.h:268:4: note: in instantiation of member function 'std::default_delete<Data>::operator()' requested here
          get_deleter()(__ptr);
          ^
8 : <source>:8:31: note: in instantiation of member function 'std::unique_ptr<Data, std::default_delete<Data> >::~unique_ptr' requested here
    std::unique_ptr<Data> m = make_me();
                              ^
3 : <source>:3:8: note: forward declaration of 'Data'
struct Data;
       ^
1 error generated.
Compiler returned: 1

But adding this line at the end makes this compiles fine:

struct Data {};

My question is why this code compiles and works when Data is declared after point of instantiation of std::unique_ptr? Seemingly, both cases should fail with the same/similar error..

Whole example on godbolt: http://ift.tt/2kU68dv