[GHC] #14488: Can't define a lens for a field with a constraint

[GHC]

#14488: Can't define a lens for a field with a constraint
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# 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:

 {{{#!hs
 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.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14488>
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