Why is int64 to int assignment not being flagged by our build as ERROR?

[kmuthukk]

Jira Link: DB-2559
https://github.com/YugaByte/yugabyte-db/blob/760cc916798ecdf7e9f906e01a8fdbaa36538df2/src/yb/consensus/log_reader.cc#L340

Status LogReader::ReadReplicatesInRange(
    const int64_t starting_at,
    const int64_t up_to,
    int64_t max_bytes_to_read,
    ReplicateMsgs* replicates) const {
...
  for (int index = starting_at; index <= up_to && !limit_exceeded; index++) {

My understanding was that we set compiler WARNINGs to be treated as ERRORs, but somehow the starting_at to index assignment isn't getting flagged.