dahbka-lis opened a new issue, #47859:
URL: https://github.com/apache/arrow/issues/47859
### Describe the bug, including details regarding any error messages,
version, and platform.
There is a bug for creating union types with empty `type_codes`. If
`fields.size() == 128` (`kMaxTypeCode `+ 1) and `type_codes` is empty,
static_cast<int8_t> returns -128 and `internal::Iota` generates an empty vector
of type codes, but the expected vector is [0, 1, 2, ..., 127], where 127 is
`kMaxTypeCode`.
Example:
```cpp
std::vector<std::shared_ptr<Field>> fields;
for (int32_t i = 0; i <= UnionType::kMaxTypeCode; i++) {
fields.push_back(field(std::to_string(i), int32()));
}
auto type = dense_union(fields); // Error
```
The validation
[here](https://github.com/apache/arrow/blob/main/cpp/src/arrow/type.cc#L1232)
will not be passed because `type_codes` is empty for `fields.size() == 128`,
but `fields` is not empty.
Otherwise:
- I can create `dense_union` type with non-empty `type_codes` vector created
by std::iota(0, 128), check the example below.
- Unions from `pyarrow` support at most 128 codes and not 127.
```cpp
std::vector<std::shared_ptr<Field>> fields;
for (int32_t i = 0; i <= UnionType::kMaxTypeCode; i++) {
fields.push_back(field(std::to_string(i), int32()));
}
std::vector<int8_t> type_codes(fields.size());
std::iota(type_codes.begin(), type_codes.end(), 0);
auto type = dense_union(fields, type_codes); // OK
```
### Component(s)
C++
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