Type checker no longer accepting code using function composition and rank-n types

The following program (reduced from Cabal code that uses HasCallStack) typechecks in GHC 8.0, but not on HEAD:

{-# LANGUAGE RankNTypes #-} 
{-# LANGUAGE ImplicitParams #-} 
module Foo where 

type T a = (?x :: Int) => a 
type M a = T (IO a) 

f :: T (T a -> a) 
f x = x 

g :: Int -> M () 
g = undefined 

h :: Int -> M () 
-- h x = f (g x) -- works on HEAD
h = f . g -- fails on HEAD, works on GHC 8.0

It's possible that this is just fall out from the recent impredicativity changes but I just wanted to make sure that this was on purpose.