presto-main/src/main/java/io/prestosql/type/TypeCoercion.java/TypeCoercion/getCommonSuperTypeForVarchar
class TypeCoercion:    private static Type getCommonSuperTypeForVarchar(VarcharType firstType, VarcharType secondType)
    {
        if (firstType.isUnbounded() || secondType.isUnbounded()) {
            return createUnboundedVarcharType();
        }

        return createVarcharType(Math.max(firstType.getBoundedLength(), secondType.getBoundedLength()));
    }