another attempt to fix the offsetof warnings

include/pybind11/common.h

@@ -191,14 +191,18 @@ NAMESPACE_BEGIN(detail)
 
 inline std::string error_string();
 
-/// PyObject wrapper around generic types
-template <typename type, typename holder_type = std::unique_ptr<type>> struct instance {
+/// Core part of the 'instance' type which POD (needed to be able to use 'offsetof')
+template <typename type> struct instance_essentials {
     PyObject_HEAD
     type *value;
     PyObject *parent;
     PyObject *weakrefs;
     bool owned : 1;
     bool constructed : 1;
+};
+
+/// PyObject wrapper around generic types, includes a special holder type that is responsible for lifetime management
+template <typename type, typename holder_type = std::unique_ptr<type>> struct instance : instance_essentials<type> {
     holder_type holder;
 };
 

include/pybind11/pybind11.h

@@ -22,9 +22,6 @@
 #  pragma GCC diagnostic ignored "-Wunused-but-set-parameter"
 #  pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 #  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
-#  if __GNUC__ >= 4
-#    pragma GCC diagnostic ignored "-Wno-invalid-offsetof"
-#  endif
 #endif
 
 #include "attr.h"
@@ -566,7 +563,7 @@ protected:
         type->ht_type.tp_dealloc = rec->dealloc;
 
         /* Support weak references (needed for the keep_alive feature) */
-        type->ht_type.tp_weaklistoffset = offsetof(instance<void>, weakrefs);
+        type->ht_type.tp_weaklistoffset = offsetof(instance_essentials<void>, weakrefs);
 
         /* Flags */
         type->ht_type.tp_flags |= Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HEAPTYPE;