Type abstraction in GHC Haskell

This might be a GHC bug. I can not see how this GHC behavior makes any sense.

This issue has nothing to do with type families, but it seems to arise from ambiguous types and typeclass constraints.

Here is a MCVE for the same issue.

{-# LANGUAGE AllowAmbiguousTypes    #-}
{-# LANGUAGE RankNTypes             #-}
{-# LANGUAGE TypeApplications       #-}

class C a where
   getInt :: Int

instance C Char where
   getInt = 42

f :: (forall a. C a => Int) -> Bool
f x = even (x @ Char)

g :: (forall a. C a => Int) -> Bool
-- g = f               -- fails
-- g h = f h           -- fails
-- g h = f getInt      -- fails
g _ = f 42             -- OK

It seems that f can not be called with any argument that actually exploits the constraint.

This is by design. It seems there is no way around using Proxys for now: https://ghc.haskell.org/trac/ghc/ticket/15119.

Edit Jul 2019: There's now a GHC proposal for this!