From @masak

<colomon> rakudo​: say 3 !+ 4
<p6eval> rakudo 125e87​: OUTPUT«0␤»
<colomon> rakudo​: say 3 !+ -3
<p6eval> rakudo 125e87​: OUTPUT«1␤»
<masak> std​: 3 !+ 4
<p6eval> std 29974​: OUTPUT«�[31m===�[0mSORRY!�[31m===�[0m␤Can't negate
+ because additive operators are not iffy enough [...] FAILED 00​:01
105m␤»
* masak submits rakudobug
<masak> not iffy enough.
<colomon> not iffy enough?!
<masak> right. that's what I said. :)
<masak> colomon​: http://irclog.perlgeek.de/perl6/2009-02-01#i_878319
<colomon> masak​: I see "iffy" there, but I don't see much in the way
of explanation that I understand?
<masak> colomon​: ok, so an iffy operator is one which produces a
boolean-ish answer.
<masak> colomon​: basically, all the logical ops, comparison ops, and %.

@moritz - Status changed from 'new' to 'resolved'

From @zhuomingliang

test added. see Raku/roast@3e93462858