[GHC] #13754: ghc-8.0.2+ rejects more instance declarations

25 May 2017

      #13754: ghc-8.0.2+ rejects more instance declarations
-------------------------------------+-------------------------------------
           Reporter:  zilinc         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Sorry for the vague title.

 For a program (sorry again but I failed to shrink it further):
 {{{
 {-# language ExistentialQuantification, KindSignatures, DataKinds, GADTs,
 UndecidableInstances, AllowAmbiguousTypes #-}

 class Pretty a where
   pretty :: a -> String
   prettyList :: [a] -> String
   prettyList = concatMap pretty

 data Def e = forall t. FunDef (Type t) (e t)

 data Nat = Z | S Nat

 data F (t :: Nat) where
   FZ :: F (S Z)
   FS :: F n -> F (S n)

 data Type (t :: Nat) = TVar (F t)

 data Expr (t :: Nat) e = Fun (Type t)
                        | App (e t) (e t)

 data TE t = TE (Type t) (Expr t TE)

 -- all methods left undefined, as they're insignificant
 instance Pretty t => Pretty [t] where
   pretty = undefined
 instance (Pretty (e t)) => Pretty (Def e) where
   pretty (FunDef t e) = undefined
 instance Pretty (Type t) where
   pretty (TVar t) = undefined
 instance Pretty (e t) => Pretty (Expr t e) where
   pretty (Fun t) = undefined
   pretty (App e1 e2) = undefined
 instance Pretty (TE t) where
   pretty (TE t e) = undefined
 }}}

 ghc-8.0.1 is happy with it, whereas later versions (tested with 8.0.2 and
 8.2.1-rc2) reject it. The error msg is:
 {{{
 GHCInstance1.hs:27:10: error:
     • Could not deduce (Pretty (e t0))
         arising from a use of ‘Main.$dmprettyList’
       from the context: Pretty (e t)
         bound by the instance declaration at GHCInstance1.hs:27:10-41
       The type variable ‘t0’ is ambiguous
       Relevant bindings include
         prettyList :: [Def e] -> String (bound at GHCInstance1.hs:27:10)
     • In the expression: Main.$dmprettyList @Def e
       In an equation for ‘prettyList’:
           prettyList = Main.$dmprettyList @Def e
       In the instance declaration for ‘Pretty (Def e)’
 }}}

 If I comment out the `prettyList` method from the class definition, then
 all happy. I'm not sure if it's a bug in 8.0.1 or a regression (and hard
 to reason about the behaviours). If it was the later, how should I migrate
 from 8.0.1? The `Pretty` class is a library (e.g. ansi-wl-pprint) so no
 way I can change it.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13754
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