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offsetofGCC implements for both C and C++ a syntactic extension to implement
the offsetof macro.
primary:
        "__builtin_offsetof" "(" typename "," offsetof_member_designator ")"
offsetof_member_designator:
          identifier
        | offsetof_member_designator "." identifier
        | offsetof_member_designator "[" expr "]"
This extension is sufficient such that
#define offsetof(type, member) __builtin_offsetof (type, member)
is a suitable definition of the offsetof macro.  In C++, type
may be dependent.  In either case, member may consist of a single
identifier, or a sequence of member accesses and array references.