why is sim_addr_range_hit_p tagged inline?

In sim/sim-arange.h:

  /* Return non-zero if ADDR is in range AR, traversing the entire tree.
     If no range is specified, that is defined to mean "everything".  */
  extern INLINE int
  sim_addr_range_hit_p (ADDR_RANGE * /*ar*/, address_word /*addr*/);
  #define ADDR_RANGE_HIT_P(ar, addr) \
    ((ar)->range_tree == NULL || sim_addr_range_hit_p ((ar), (addr)))

Why is the INLINE there?  sim_addr_range_hit_p() is only defined in
sim-arange.c