The definition of offset_of() in globalDefinitions_*.hpp returns a type size_t which in all the uses in header files does an implicit conversion down to an 'int'. The 'int' is large enough to hold offset_of things and there's a function and comment in src/hotspot/share/utilities/sizes.hpp:

    // Use the following #define to get C++ field member offsets
    #define byte_offset_of(klass,field) in_ByteSize((int)offset_of(klass, field))

Fixing the header file uses of offset_of avoids -Wconversion warnings when also disabling -Werror from 1,621,150 to 863,682 lines of warnings.