[perl #128586] [BUG] andthen doesn't return undefined LHS

[Zoffix Znet via RT]

On Sat, 09 Jul 2016 11:03:04 -0700, pmichaud wrote:
> Per S03, &infix:<andthen> should return the first argument indicating
> failure (undefined).  On current Rakudo:
> 
> 18:00 <pmichaud> m:  say (Int andthen 3)
> 18:00 <camelia> rakudo-moar 959cd3: OUTPUT«()␤»
> 18:00 <pmichaud> m:  say (Int andthen 3).WHAT
> 18:00 <camelia> rakudo-moar 959cd3: OUTPUT«(Slip)␤»
> 
> Per S03, the answer should be OUTPUT«(Int)␤».
> 
> Pm

FWIW, when I was fixing another bug, TimToady confirmed `andthen` returns an 
Empty by design, so that it works with list comprehensions: 
https://irclog.perlgeek.de/perl6-dev/2016-11-11#i_13550043