Program involving existentials and type class constraint gets rejected

The following example from https://stackoverflow.com/a/50160423/388010 demonstrates some code involving type classes and existentials that I'd love for it to pass the type checker:

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

module Foo where

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

This complains with the following error:

test.hs:17:5: error:
    • Could not deduce (C a0)
      from the context: C a
        bound by a type expected by the context:
                   forall a. C a => Int
        at test.hs:17:5
      The type variable ‘a0’ is ambiguous
      These potential instance exist:
        instance C Char -- Defined at test.hs:10:10
    • In the expression: f
      In an equation for ‘g’: g = f
   |
17 | g = f                  -- fails
   |     ^

Is this expected behavior or a bug? I understand why the program leads to that particular error, but isn't there a way for the compiler to accept this?

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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