neilconway opened a new issue, #10855:
URL: https://github.com/apache/arrow-rs/issues/10855
### Describe the bug
`ToPrimitive::to_i64` for i256 returns the low 64 bits of some values that
do not fit in i64; this is reached by `cast` from `Decimal256` to signed
integers, yielding incorrect results.
### To Reproduce
```rust
use arrow_array::{cast::AsArray, types::Int64Type, Decimal256Array};
use arrow_buffer::i256;
use arrow_cast::{cast_with_options, CastOptions};
use arrow_schema::DataType;
use num_traits::ToPrimitive;
// 2^64 + 5 = 18446744073709551621, which does not fit in i64
let v = i256::from_i128((1i128 << 64) + 5);
v.to_i64(); // Some(5) — expected None
let arr = Decimal256Array::from(vec![v])
.with_precision_and_scale(76, 0)
.unwrap();
let strict = CastOptions { safe: false, ..Default::default() };
let out = cast_with_options(&arr, &DataType::Int64, &strict).unwrap();
out.as_primitive::<Int64Type>().value(0); // 5 — expected an error (safe
mode gives 5 too, not null)
```
### Expected behavior
_No response_
### Additional context
_No response_
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