error: enum value `0` used more than once
  --> testdata/tags/allow_alias.proto:21:10
   |
20 |     A1 = 0;
   |          - previously used here
21 |     B1 = 0;
   |          ^ used here

error: `allow_alias` requires at least one aliasing enum value
  --> testdata/tags/allow_alias.proto:31:12
   |
31 |     option allow_alias = true;
   |            ^^^^^^^^^^^^^^^^^^

encountered 2 errors
