mbutrovich opened a new issue, #12095:
URL: https://github.com/apache/arrow-rs/issues/12095
**Describe the bug**
`PrimitiveArray::validate_decimal_precision` returns `Decimal precision
{precision} is less than scale {scale}` for any array whose scale is negative,
whatever its precision and values. Its guard compares `precision < self.scale()
as u8`, and a negative scale cast to `u8` wraps, so `-2` becomes `254`.
https://github.com/apache/arrow-rs/blob/cc58320c2da359b757814db5a9651397d829b601/arrow-array/src/array/primitive_array.rs#L1708-L1714
The type-level check `validate_decimal_precision_and_scale` guards the same
comparison with `scale > 0`:
https://github.com/apache/arrow-rs/blob/cc58320c2da359b757814db5a9651397d829b601/arrow-array/src/types.rs#L1465-L1469
The guard came in with #8396, first released in 57.0.0.
**To Reproduce**
On `main` at cc58320c2d:
```rust
use arrow_array::Decimal128Array;
fn main() {
// 1 at scale -2 is 100, which fits in precision 5
let a = Decimal128Array::from(vec![1])
.with_precision_and_scale(5, -2)
.unwrap();
println!("{:?}", a.validate_decimal_precision(5));
// Err(InvalidArgumentError("Decimal precision 5 is less than scale -2"))
// The same value at scale 2 passes
let b = Decimal128Array::from(vec![1])
.with_precision_and_scale(5, 2)
.unwrap();
println!("{:?}", b.validate_decimal_precision(5));
// Ok(())
}
```
**Expected behavior**
`Ok(())` for the first array, since its value fits precision 5. A guard of
`self.scale() > 0 && precision < self.scale() as u8` would match
`validate_decimal_precision_and_scale`.
**Additional context**
Property-based tests found it on a generated `Decimal128(5, -2)` array.
Checking each value with `DecimalType::validate_decimal_precision` works around
it.
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