This might be a problem for backports, as update project might need to continue being buildable with older GCCs.

Reliably fails like this:

* For target support_native_java.desktop_libfontmanager_hb-draw.o:
In file included from src/java.desktop/share/native/libharfbuzz/hb.hh:473:0,
                 from src/java.desktop/share/native/libharfbuzz/hb-draw.cc:25:
src/java.desktop/share/native/libharfbuzz/hb-cplusplus.hh:170:13: error: specialization of 'template<class _Tp> struct std::hash' in different namespace [-fpermissive]
 struct std::hash<hb::shared_ptr<T>>
             ^~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/stdexcept:39,
                 from /usr/include/c++/6/array:39,
                 from /usr/include/c++/6/tuple:39,
                 from /usr/include/c++/6/functional:55,
                 from src/java.desktop/share/native/libharfbuzz/hb-cplusplus.hh:35,
                 from src/java.desktop/share/native/libharfbuzz/hb.hh:473,
                 from src/java.desktop/share/native/libharfbuzz/hb-draw.cc:25:
/usr/include/c++/6/bits/functional_hash.h:58:12: error: from definition of 'template<class _Tp> struct std::hash' [-fpermissive]
   ... (rest of output omitted)


This might be a * For target support_native_java.desktop_libfontmanager_hb-draw.o:
In file included from src/java.desktop/share/native/libharfbuzz/hb.hh:473:0,
                 from src/java.desktop/share/native/libharfbuzz/hb-draw.cc:25:
src/java.desktop/share/native/libharfbuzz/hb-cplusplus.hh:170:13: error: specialization of 'template<class _Tp> struct std::hash' in different namespace [-fpermissive]
 struct std::hash<hb::shared_ptr<T>>
             ^~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/stdexcept:39,
                 from /usr/include/c++/6/array:39,
                 from /usr/include/c++/6/tuple:39,
                 from /usr/include/c++/6/functional:55,
                 from src/java.desktop/share/native/libharfbuzz/hb-cplusplus.hh:35,
                 from src/java.desktop/share/native/libharfbuzz/hb.hh:473,
                 from src/java.desktop/share/native/libharfbuzz/hb-draw.cc:25:
/usr/include/c++/6/bits/functional_hash.h:58:12: error: from definition of 'template<class _Tp> struct std::hash' [-fpermissive]
   ... (rest of output omitted)