[GHC] #11480: UndecidableSuperClasses causes the compiler to spin with UndecidableInstances

[GHC]

#11480: UndecidableSuperClasses causes the compiler to spin with
UndecidableInstances
-------------------------------------+-------------------------------------
        Reporter:  ekmett            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:  PolyKinds,
      Resolution:                    |  UndecidableSuperClasses
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10318            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 There also seems to be a bad interaction with the `?callStack` machinery.

 Here is a differently modified test case:

 {{{
 {-# language KindSignatures, PolyKinds, DataKinds, TypeFamilies,
 RankNTypes, NoImplicitPrelude, FlexibleContexts, MultiParamTypeClasses,
 GADTs, ConstraintKinds, FlexibleInstances, TypeOperators,
 ScopedTypeVariables, UndecidableSuperClasses, FunctionalDependencies #-}

 import GHC.Types (Constraint)
 import qualified Prelude

 data Dict p where
   Dict :: p => Dict p

 newtype p :- q = Sub (p => Dict q)

 data Nat (c :: i -> i -> *) (d :: j -> j -> *) (f :: i -> j) (g :: i -> j)

 class Functor p (Nat p (->)) p => Category (p :: i -> i -> *) where
   type Ob p :: i -> Constraint

 class (Category dom, Category cod) => Functor (dom :: i -> i -> *) (cod ::
 j -> j -> *) (f :: i -> j) | f -> dom cod

 bug :: Functor c d f => Ob c a :- Ob d (f a)
 bug = Prelude.undefined
 }}}

 I attempted to place `undefined` there as a placeholder while I worked on
 the surrounding code, but compiling `bug` causes

 {{{
     solveWanteds: too many iterations (limit = 4)
       Unsolved: WC {wc_simple =
                       [W] $dIP_a15a ::
 ?callStack::GHC.Stack.Types.CallStack (CDictCan)}
       New superclasses found
       Set limit with -fconstraint-solver-iterations=n; n=0 for no limit
 }}}

 Raising the limit gets me right back to the same unsolved constraint.

 Without the class cycle, we don't spin forever trying to find a
 `?callStack`.

 It seems odd that looking for `?callStack` would cause us to unroll
 superclasses though, as implicit parameters can't be a superclass of any
 class.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11480#comment:2>
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