In file included from /usr/include/c++/6/iostream:38:0,
                 from /in/foo.cc:17:
/usr/include/c++/6/sstream: In function 'int main()':
/usr/include/c++/6/sstream:41:15: error: expected '=' before '__attribute__'
 namespace std _GLIBCXX_VISIBILITY(default)
               ^
/usr/include/c++/6/sstream:41:15: error: expected identifier before '__attribute__'
/usr/include/c++/6/sstream:41:15: error: expected ';' before '__attribute__'
/usr/include/c++/6/sstream:41:15: error: expected primary-expression before '__attribute__'
/usr/include/c++/6/bits/sstream.tcc:39:15: error: expected '=' before '__attribute__'
 namespace std _GLIBCXX_VISIBILITY(default)
               ^
/usr/include/c++/6/bits/sstream.tcc:39:15: error: expected identifier before '__attribute__'
/usr/include/c++/6/bits/sstream.tcc:39:15: error: expected ';' before '__attribute__'
/usr/include/c++/6/bits/sstream.tcc:39:15: error: expected primary-expression before '__attribute__'
/in/foo.cc:38:21: error: aggregate 'std::ostringstream oss' has incomplete type and cannot be defined
  std::ostringstream oss;
                     ^~~