# QuantifiedConstraints: Implication constraints with type families don't work

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
```