diff --git a/gc/default.c b/gc/default.c
index 65aacb70a3..4df9a8b9fd 100644
--- a/gc/default.c
+++ b/gc/default.c
@@ -1683,9 +1683,6 @@ heap_page_add_freeobj(rb_objspace_t *objspace, struct heap_page *page, VALUE obj
     gc_report(3, objspace, "heap_page_add_freeobj: add %p to freelist\n", (void *)obj);
 }
 
-static size_t
-objspace_available_slots(rb_objspace_t *objspace);
-
 static void
 size_pool_allocatable_slots_expand(rb_objspace_t *objspace,
         rb_size_pool_t *size_pool, size_t free_slots, size_t total_slots)