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This may be related to #14860, but I think it's different.

The following code fails to compile:

{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE ConstraintKinds #-}

import Prelude()
import Data.Kind

data Dict c = c => Dict

type family F a :: Constraint

foo :: forall a b. (a => F b, a) => Dict (F b)
foo = Dict

• Could not deduce: F b arising from a use of ‘Dict’
  from the context: (a => F b, a)

Yet the following all do compile:

bar :: forall a. F a => Dict (F a)
bar = Dict

baz :: forall a b. (a => b, a) => Dict b
baz = Dict

qux :: forall a b c. (a => c, a, c ~ F b) => Dict (F b)
qux = Dict

It seems that a wanted F b can be solved with a given F b, but not with a given a => F b, which is bizarre. The fact that qux still works is also strange, as it means that you get a different result if you first simplify by substituting c = F b.

As a more practical example, the following similarly fails to compile, due to the Cod f type family:

-- in addition to the above extensions
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}

class Ob p a

class (forall a. Ob (Dom f) a => Ob (Cod f) (f a)) => Functor f where
  type Dom f
  type Cod f

liftOb :: forall f a. (Functor f, Ob (Dom f) a) => Dict (Ob (Cod f) (f a))
liftOb = Dict

While a version which uses fundeps instead does compile:

class (forall a. Ob dom a => Ob cod (f a)) => Functor dom cod f | f -> dom cod

liftOb :: forall f a dom cod. (Functor dom cod f, Ob dom a) => Dict (Ob cod (f a))
liftOb = Dict