error: use of synthetic map entry type
  --> testdata/fields/maps/entry_misuse.proto:21:14
   |
20 |     map<int32, int32> foo_bar = 1;
   |     ----------------- synthesized by this type
21 |     repeated FooBarEntry foo_bars = 2;
   |              ^^^^^^^^^^^ referenced here
   |
   = help: despite having a user-visible symbol, map entry types cannot be used
           as field types

encountered 1 error
