Clearify status of ImpredicativeTypes Extension
There seem to be many statements about this extension and its problems floating around in the internet. But a user of GHC will first fall into a possibly large number of traps and problems before he finds them.
I think it would help, if the GHC manual https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/other-type-extensions.html#impredicative-polymorphism and the compiler warned about this extension (instead of suggesting it!).