pitrou opened a new issue, #35942:
URL: https://github.com/apache/arrow/issues/35942
### Describe the bug, including details regarding any error messages,
version, and platform.
It seems the expected result from converting a decimal to real may be off by
one ULP:
```pycon
>>> a = pa.array(['999999999999999.9']).cast(pa.decimal128(17,1))
>>> a
<pyarrow.lib.Decimal128Array object at 0x7f8ec6f3f6a0>
[
999999999999999.9
]
>>> a.cast(pa.float64())
<pyarrow.lib.DoubleArray object at 0x7f8fa4a88ee0>
[
1e+15
]
>>> expected = 999999999999999.9
>>> actual = a.cast(pa.float64())[0].as_py()
>>> abs(expected-actual)/expected
1.25e-16
>>> abs(expected-actual) == expected - math.nextafter(expected, 0)
True # Exactly one ULP
```
This is much less sever
### Component(s)
C++
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