[Haskell-cafe] Non-existing types in existential quantification?

Given the following code, that is accepted by GHC:

data Exist = forall a. Exist a

exist :: Exist exist = Exist undefined

What type has the 'undefined' ? So far I assumed that at runtime all objects have a concrete type. This seems not to be true. I can also write

data ExistList = forall a. ExistList [a]

exist :: ExistList exist = ExistList []

where I do not need an 'undefined'.