This is xnu-8019. See this file in:
//
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#ifndef XNU_LIBKERN_LIBKERN_CXX_BOUNDED_PTR_FWD_H
#define XNU_LIBKERN_LIBKERN_CXX_BOUNDED_PTR_FWD_H

#if !TAPI

namespace libkern {
template <typename T, typename TrappingPolicy>
struct __attribute__((trivial_abi)) bounded_ptr;
} // end namespace libkern

#endif /* !TAPI */

#endif // !XNU_LIBKERN_LIBKERN_CXX_BOUNDED_PTR_FWD_H