Cannot make existential type instance of class with fundep

[guest]

There seems to be a problem when combining FunDeps and Existential Types. The code below (works on ghci) causes an error:

- ERROR "Existential.hs":8 - Existentially quantified variable escapes from pattern FooObj a.

class Foo a b | a -> b where foo :: a -> (String,b)

data FooA = A
instance Foo FooA String where foo _  = ("tyA","A-specific")

data FooObj = forall a b. Foo a b => FooObj a 
instance Foo FooObj () where foo (FooObj a) = (fst (foo a),()) -- !!!

best regards, benedikt

[zlgdgzl]

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