[perl #130431] [REGRESSION] Subclassing an Int no longer works flawlessly (my Foo $a .= new(42))

[via RT]

# New Ticket Created by  Aleks-Daniel Jakimenko-Aleksejev 
# Please include the string:  [perl #130431]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=130431 >


Code:
class Mebibytes is Int { method Str { (self/1024/1024).Int ~ "M" } }; my  
Mebibytes $a .= new(183456783); $a.say; # fancy enough?


Result (2016.09):
174M


Result (HEAD):
Type check failed in assignment to $a; expected Mebibytes but got Int 
(183456783)
in block <unit> at /tmp/4Vz0jddWQp line 1


Bisectable points to 
https://github.com/rakudo/rakudo/commit/798c2e2dca34e5cf93da99d1f401eb161efca854


<Zoffix> AlexDaniel, please rakudobug it. I'll take a look tomorrow. At least 
half of 
https://github.com/rakudo/rakudo/commit/798c2e2dca34e5cf93da99d1f401eb161efca854
 is wrong, because that comment lies; it doesn't give a "decoupled" Int