Can't define a lens for a field with a constraint

{-# LANGUAGE RankNTypes #-}

type Lens' s a = forall f. Functor f => (a -> f a) -> s -> f s

data T a = MkT { _tfield :: Eq a => a }

tfield :: Eq a => Lens' (T a) a
tfield f t = MkT <$> f (_tfield t)

This code compiles with GHC 8.2.1. On GHC 8.0.2 the following error is reported:

tfield.hs:8:22: error:
    • Couldn't match type ‘a’ with ‘Eq a => a’
      ‘a’ is a rigid type variable bound by
        the type signature for:
          tfield :: forall a. Eq a => Lens' (T a) a
        at tfield.hs:7:11
      Expected type: f (Eq a => a)
        Actual type: f a
    • In the second argument of ‘(<$>)’, namely ‘f (_tfield t)’
      In the expression: MkT <$> f (_tfield t)
      In an equation for ‘tfield’: tfield f t = MkT <$> f (_tfield t)
    • Relevant bindings include
        t :: T a (bound at tfield.hs:8:10)
        f :: a -> f a (bound at tfield.hs:8:8)
        tfield :: Lens' (T a) a (bound at tfield.hs:8:1)

I could not find a relevant GHC ticket. Unless it's a known issue, I am going to create a Phab Diff with this code as a test case to avoid a regression in the future.

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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