The BSON_ASSERT macro uses typedef tricks to force a compilation error if the static invariant it asserts is false. Unfortunately the symbols it typedefs are named only using the line number on which BSON_ASSERT is used. E.g. if there is a BSON_ASSERT on line 112, then a symbol static_assert_test_112 is involved in checking the invariant. If BSON_ASSERT is used in two files on the same line number in each, a pedantic compiler errors, "error: redefinition of typedef 'static_assert_test_112'".

See for example:

https://evergreen.mongodb.com/task_log_raw/mongo_c_driver_clang35_debug_compile_nosasl_nossl_patch_294976a086ce1a1d7c8dc904d8ba2503ae7e648e_59b413ede3c3311323000252_17_09_09_16_16_45/0?type=T#L907

To solve, make a new macro BSON_STATIC_ASSERT2 that allows a custom string prefix:

BSON_STATIC_ASSERT2 (sizeof_counter_slots,
                     sizeof (mongoc_counter_slots_t) == 64);