[ruby-core:114196] [Ruby master Bug#19318] Float#round rounds incorrectly for some cases

Issue #19318 has been updated by nagachika (Tomoyuki Chikanaga). Backport changed from 2.7: UNKNOWN, 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED to 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONTNEED I don't think the fixes for this kind of long standing bugs should not be backported to stable branches. usa-san, what is your opinion? ---------------------------------------- Bug #19318: Float#round rounds incorrectly for some cases https://bugs.ruby-lang.org/issues/19318#change-103883 * Author: Eregon (Benoit Daloze) * Status: Closed * Priority: Normal * Backport: 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONTNEED ---------------------------------------- This was discovered by @aardvark179. The following spec in `spec/ruby/core/float/round_spec.rb` fails on CRuby: ```ruby ruby_bug "", ""..."3.3" do # These numbers are neighbouring floating point numbers round a # precise value. They test that the rounding modes work correctly # round that value and precision is not lost which might cause # incorrect results. it "does not lose precision during the rounding process" do 767573.1875850001.round(5, half: nil).should eql(767573.18759) 767573.1875850001.round(5, half: :up).should eql(767573.18759) 767573.1875850001.round(5, half: :down).should eql(767573.18759) 767573.1875850001.round(5, half: :even).should eql(767573.18759) -767573.1875850001.round(5, half: nil).should eql(-767573.18759) -767573.1875850001.round(5, half: :up).should eql(-767573.18759) -767573.1875850001.round(5, half: :down).should eql(-767573.18759) -767573.1875850001.round(5, half: :even).should eql(-767573.18759) 767573.187585.round(5, half: nil).should eql(767573.18759) 767573.187585.round(5, half: :up).should eql(767573.18759) 767573.187585.round(5, half: :down).should eql(767573.18758) 767573.187585.round(5, half: :even).should eql(767573.18758) -767573.187585.round(5, half: nil).should eql(-767573.18759) -767573.187585.round(5, half: :up).should eql(-767573.18759) -767573.187585.round(5, half: :down).should eql(-767573.18758) -767573.187585.round(5, half: :even).should eql(-767573.18758) 767573.1875849998.round(5, half: nil).should eql(767573.18758) 767573.1875849998.round(5, half: :up).should eql(767573.18758) 767573.1875849998.round(5, half: :down).should eql(767573.18758) 767573.1875849998.round(5, half: :even).should eql(767573.18758) -767573.1875849998.round(5, half: nil).should eql(-767573.18758) -767573.1875849998.round(5, half: :up).should eql(-767573.18758) -767573.1875849998.round(5, half: :down).should eql(-767573.18758) -767573.1875849998.round(5, half: :even).should eql(-767573.18758) end end ``` Yet this test to the best of our knowledge is correct. This was fixed on master by @mrkn in https://github.com/ruby/ruby/pull/7023 (thanks!). The question is should we backport this? I think yes. -- https://bugs.ruby-lang.org/