[Haskell-cafe] ghc-9 needs type declaration where ghc-8 didn't (with RankNTypes)

[Johannes Waldmann]

> The 9.0 upgrade may help: 
> https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0

Thanks for that pointer!

So, what I observed is an effect of
https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.0#simplified-subsumption
?
I was browsing that, but could not make the connection.
"nested foralls" are mentioned only together with
GADT/instance/deriving, none of which is in my example.

- J.