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ghc-8.0.1-rc4: unification false positive?

    • Open Issue created by Serge Kosyrev @_deepfire 
    {-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UnicodeSyntax #-}

module Foo () where

type family   TF x ∷ *
data              ADT x
type instance TF (ADT x) = x

class (a ~ ADT (TF a)) ⇒ TC2 a b | a → b

data Forget = ∀ a b. TC2 a b ⇒ Forget a -- ~ Forget (ADT (TF a)) 

data PhantomF a b = Constr Forget       -- ~ Constr (Forget (ADT (TF a)))

f ∷ ∀ a b. TC2 a b ⇒ ADT (TF a) → [Forget]
f _ = case ((undefined) ∷ (PhantomF a b)) of
        Constr m → [Forget m]

-- Here GHC 8.0.1-rc4 unifies, whereas GHC 7.10.3 (properly?) fails with:

-- ghc8-unification-false-positive.hs:20:21:
--     Couldn't match type ‘Forget’ with ‘ADT (TF Forget)’
--     In the expression: Forget m
--     In the expression: [Forget m]
--     In a case alternative: Constr m -> [Forget m]
    Trac metadata
    Trac field Value
    Version 8.0.1-rc4
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component Compiler
    Test case
    Differential revisions
    BlockedBy
    Related
    Blocking
    CC
    Operating system
    Architecture
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