[GHC] #14846: Renamer hangs (because of -XInstanceSigs?)

[GHC]

#14846: Renamer hangs (because of -XInstanceSigs?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:  InstanceSigs
                                     |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Old description:

> {{{#!hs
> {-# Language RankNTypes, TypeInType, EmptyCase, GADTs, FlexibleInstances,
> ConstraintKinds, UndecidableInstances, AllowAmbiguousTypes, InstanceSigs
> #-}
>
> import Data.Kind
> import Data.Proxy
>
> type Cat ob = ob -> ob -> Type
>
> data Struct :: (k -> Constraint) -> Type where
>   S :: Proxy (a::k) -> Struct (cls::k -> Constraint)
>
> type Structured a cls = (S ('Proxy :: Proxy a)::Struct cls)
>
> data AStruct :: Struct cls -> Type where
>   AStruct :: cls a => AStruct (Structured a cls)
>
> class StructI (structured::Struct (cls :: k -> Constraint)) where
>   struct :: AStruct structured
>
> instance (Structured xx cls ~ structured, cls xx) => StructI structured
> where
>   struct :: AStruct (Structured xx cls)
>   struct = AStruct
>
> data Hom :: Cat k -> Cat (Struct cls) where
>
> class Category (cat::Cat ob) where
>   i :: StructI a => ríki a a
>
> instance Category ríki => Category (Hom ríki :: Cat (Struct cls)) where
>   i :: forall a. StructI a => Hom ríki a a
>   i = case struct :: AStruct (Structured a cls) of
> }}}
>
> Running on 8.2.1 and 8.5.20180105 both loop until interrupted
>
> {{{
> $ ghci -ignore-dot-ghci 199.hs
> GHCi, version 8.5.20180105: http://www.haskell.org/ghc/  :? for help
> [1 of 1] Compiling Main             ( 199.hs, interpreted )
> ^CInterrupted.
> >
> >
> }}}

New description:

 {{{#!hs
 {-# Language RankNTypes, TypeInType, EmptyCase, GADTs, FlexibleInstances,
 ConstraintKinds, Undecida
 bleInstances, AllowAmbiguousTypes, InstanceSigs, ScopedTypeVariables #-}

 import Data.Kind
 import Data.Proxy

 type Cat ob = ob -> ob -> Type

 data Struct :: (k -> Constraint) -> Type where
   S :: Proxy (a::k) -> Struct (cls::k -> Constraint)

 type Structured a cls = (S ('Proxy :: Proxy a)::Struct cls)

 data AStruct :: Struct cls -> Type where
   AStruct :: cls a => AStruct (Structured a cls)

 class StructI (structured::Struct (cls :: k -> Constraint)) where
   struct :: AStruct structured

 instance (Structured xx cls ~ structured, cls xx) => StructI structured
 where
   struct :: AStruct (Structured xx cls)
   struct = AStruct

 data Hom :: Cat k -> Cat (Struct cls) where

 class Category (cat::Cat ob) where
   i :: StructI a => ríki a a

 instance Category ríki => Category (Hom ríki :: Cat (Struct cls)) where
   -- Commenting out this instance signature makes the issue go away
   i :: forall a. StructI a => Hom ríki a a
   i = case struct :: AStruct (Structured a cls) of
 }}}

 Running on 8.2.1 and 8.5.20180105 both loop until interrupted

 {{{
 $ ghci -ignore-dot-ghci 199.hs
 GHCi, version 8.5.20180105: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( 199.hs, interpreted )
 ^CInterrupted.
 >
 >
 }}}

--

Comment (by RyanGlScott):

 Thanks. I've updated the original program to reflect this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14846#comment:3>
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