[ruby-core:78235] [Ruby trunk Bug#9979][Feedback] Regression: BigDecimal::ROUND_HALF_DOWN/ROUND_HALF_EVEN behave incorrectly

Issue #9979 has been updated by Kenta Murata.

Status changed from Assigned to Feedback

I think the current behavior is the correct one.

```
$ ruby -rbigdecimal -ve 'p Gem::Dependency.new("bigdecimal").to_spec.version.to_s; puts ("0.50"..."0.60").map {|x| [x, BigDecimal(x).round(0, BigDecimal::ROUND_HALF_DOWN).to_s("F")].join(" -> ") }'
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]
"1.2.8"
0.50 -> 0.0
0.51 -> 1.0
0.52 -> 1.0
0.53 -> 1.0
0.54 -> 1.0
0.55 -> 1.0
0.56 -> 1.0
0.57 -> 1.0
0.58 -> 1.0
0.59 -> 1.0

$ ruby -rbigdecimal -ve 'p Gem::Dependency.new("bigdecimal").to_spec.version.to_s; puts ("0.50"..."0.60").map {|x| [x, BigDecimal(x).round(0, BigDecimal::ROUND_HALF_EVEN).to_s("F")].join(" -> ") }'
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]
"1.2.8"
0.50 -> 0.0
0.51 -> 1.0
0.52 -> 1.0
0.53 -> 1.0
0.54 -> 1.0
0.55 -> 1.0
0.56 -> 1.0
0.57 -> 1.0
0.58 -> 1.0
0.59 -> 1.0

$ ruby -rbigdecimal -ve 'p Gem::Dependency.new("bigdecimal").to_spec.version.to_s; puts ("1.50"..."1.60").map {|x| [x, BigDecimal(x).round(0, BigDecimal::ROUND_HALF_EVEN).to_s("F")].join(" -> ") }'
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]
"1.2.8"
1.50 -> 2.0
1.51 -> 2.0
1.52 -> 2.0
1.53 -> 2.0
1.54 -> 2.0
1.55 -> 2.0
1.56 -> 2.0
1.57 -> 2.0
1.58 -> 2.0
1.59 -> 2.0
```

Joseph, please show the wrong examples.

----------------------------------------
Bug #9979: Regression: BigDecimal::ROUND_HALF_DOWN/ROUND_HALF_EVEN behave incorrectly
https://bugs.ruby-lang.org/issues/9979#change-61604

* Author: Joseph Leniston
* Status: Feedback
* Priority: Normal
* Assignee: Kenta Murata
* ruby -v: 2.1.2
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
The incorrect behaviour is that all fractional values between 0.5 (inclusive) and 0.6 (non-inclusive) are subject to the rounding policy for 'half', whereas it should only be applied for fractional values exactly equal to 0.5.



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