boost fails to build with Python 3.10.0a2.

In file included from /usr/include/python3.10/pytime.h:6,
                 from /usr/include/python3.10/Python.h:85,
                 from ./boost/python/detail/wrap_python.hpp:178,
                 from ./boost/python/detail/prefix.hpp:13,
                 from ./boost/python/object_core.hpp:10,
                 from ./boost/python/object/enum_base.hpp:8,
                 from libs/python/src/object/enum.cpp:6:
libs/python/src/object/enum.cpp: In function 'boost::python::api::object boost::python::objects::{anonymous}::new_enum_type(const char*, const char*)':
/usr/include/python3.10/object.h:140:29: error: lvalue required as left operand of assignment
  140 | #define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/enum.cpp:156:11: note: in expansion of macro 'Py_TYPE'
  156 |           Py_TYPE(&enum_type_object) = incref(&PyType_Type);
      |           ^~~~~~~
gcc.compile.c++ serial/boost/bin.v2/libs/python/build/gcc-10/release/debug-symbols-on/pch-off/python-3.10/threading-multi/visibility-hidden/object/class.o

    "g++"  -fvisibility-inlines-hidden -fPIC -m64 -pthread -g -fvisibility=hidden -O2 -flto=auto -ffat-lto-objects -fexceptions -g -grecord-gcc-switches -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -Wp,-D_GLIBCXX_ASSERTIONS -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -fstack-protector-strong -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1  -m64 -mtune=generic -fasynchronous-unwind-tables -fstack-clash-protection -fcf-protection -fno-strict-aliasing -Wno-unused-local-typedefs -Wno-deprecated-declarations  -DBOOST_ALL_NO_LIB=1 -DBOOST_PYTHON_SOURCE -DNDEBUG  -I"." -I"/usr/include/python3.10" -c -o "serial/boost/bin.v2/libs/python/build/gcc-10/release/debug-symbols-on/pch-off/python-3.10/threading-multi/visibility-hidden/object/class.o" "libs/python/src/object/class.cpp"

In file included from /usr/include/python3.10/pytime.h:6,
                 from /usr/include/python3.10/Python.h:85,
                 from ./boost/python/detail/wrap_python.hpp:178,
                 from ./boost/python/detail/prefix.hpp:13,
                 from libs/python/src/object/class.cpp:6:
libs/python/src/object/class.cpp: In function 'PyObject* boost::python::objects::static_data()':
/usr/include/python3.10/object.h:140:29: error: lvalue required as left operand of assignment
  140 | #define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/class.cpp:211:11: note: in expansion of macro 'Py_TYPE'
  211 |           Py_TYPE(&static_data_object) = &PyType_Type;
      |           ^~~~~~~
libs/python/src/object/class.cpp: In function 'boost::python::type_handle boost::python::objects::class_metatype()':
/usr/include/python3.10/object.h:140:29: error: lvalue required as left operand of assignment
  140 | #define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/class.cpp:319:11: note: in expansion of macro 'Py_TYPE'
  319 |           Py_TYPE(&class_metatype_object) = &PyType_Type;
      |           ^~~~~~~
libs/python/src/object/class.cpp: In function 'PyObject* boost::python::objects::instance_new(PyTypeObject*, PyObject*, PyObject*)':
/usr/include/python3.10/object.h:134:29: error: lvalue required as left operand of assignment
  134 | #define Py_SIZE(ob) _Py_SIZE(_PyVarObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/class.cpp:378:15: note: in expansion of macro 'Py_SIZE'
  378 |               Py_SIZE(result) =
      |               ^~~~~~~
libs/python/src/object/class.cpp: In function 'boost::python::type_handle boost::python::objects::class_type()':
/usr/include/python3.10/object.h:140:29: error: lvalue required as left operand of assignment
  140 | #define Py_TYPE(ob) _Py_TYPE(_PyObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/class.cpp:473:11: note: in expansion of macro 'Py_TYPE'
  473 |           Py_TYPE(&class_type_object) = incref(class_metatype().get());
      |           ^~~~~~~
libs/python/src/object/class.cpp: In static member function 'static void* boost::python::instance_holder::allocate(PyObject*, std::size_t, std::size_t)':
/usr/include/python3.10/object.h:134:29: error: lvalue required as left operand of assignment
  134 | #define Py_SIZE(ob) _Py_SIZE(_PyVarObject_CAST_CONST(ob))
      |                     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
libs/python/src/object/class.cpp:742:9: note: in expansion of macro 'Py_SIZE'
  742 |         Py_SIZE(self) = holder_offset;
      |         ^~~~~~~
...failed updating 2 targets...
error: Bad exit status from /var/tmp/rpm-tmp.oihGUt (%build)

https://docs.python.org/3.10/whatsnew/3.10.html#id2

Since Py_TYPE() is changed to the inline static function, Py_TYPE(obj) = new_type must be replaced with Py_SET_TYPE(obj, new_type): see Py_SET_TYPE() (available since Python 3.9). For backward compatibility, this macro can be used:

#if PY_VERSION_HEX < 0x030900A4
#  define Py_SET_TYPE(obj, type) ((Py_TYPE(obj) = (type)), (void)0)
#endif
https://bugs.python.org/issue39573

For the build logs, see:
https://copr-be.cloud.fedoraproject.org/results/@python/python3.10/fedora-rawhide-x86_64/01756932-boost/

For all our attempts to build boost with Python 3.10, see:
https://copr.fedorainfracloud.org/coprs/g/python/python3.10/package/boost/

Testing and mass rebuild of packages is happening in copr. You can follow these instructions to test locally in mock if your package builds with Python 3.10:
https://copr.fedorainfracloud.org/coprs/g/python/python3.10/

Let us know here if you have any questions.

Python 3.10 will be included in Fedora 35. To make that update smoother, we're building Fedora packages with early pre-releases of Python 3.10.
A build failure prevents us from testing all dependent packages (transitive [Build]Requires), so if this package is required a lot, it's important for us to get it fixed soon.
We'd appreciate help from the people who know this package best, but if you don't want to work on this now, let us know so we can try to work around it on our side.