TClass topClass = tclass(instances[field]);
TClass botClass = tclass(botInstance);
TClass commonTClass = registry.getCastsResolver().commonTClass(topClass, botClass);
if (commonTClass == null) {
throw new AkibanInternalException("no common type found found between row " + (rownum-1)
+ " and " + rownum + " at field " + field);
}
// The two rows have different TClasses at this index, so we'll need at least one of them to
// be casted. Also the common class will be the widest comparable.
needCasts.set(field);