[perl #126818] on "is cached" subs: getlex: outer index out of range

[via RT]

# New Ticket Created by  Zoffix Znet 
# Please include the string:  [perl #126818]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=126818 >


Steps to reproduce:
#############################################
$ cat > Foo.pm;
unit module Foo;
our sub foo () is cached { 42 }
$ perl6 -I. -MFoo -e 'say Foo::foo'
getlex: outer index out of range
  in any enter at gen/moar/m-Metamodel.nqp:3763
  in block <unit> at -e:1
#############################################

The problem goes away if "is cached" trait is removed.