Memoize in between of two JOIN nodes

Playing with memoisation, I found the case where the Memoize is put over 
a JOIN node (see attachment).
I recall a discussion we had with Richard in which he mentioned [1] that 
this feature is still not implemented and is hard to design.
I'm not sure, but may it be a sign of a potential bug?

[1] 
https://www.postgresql.org/message-id/CAMbWs4_CQR9ikyAhjH0CwzCdp%2BFcRAeoVsi5Oohd7s%3Dc5RZABg%40mail.gmail.com

-- 
regards, Andrei Lepikhov