[GHC] #15552: Infinite loop/panic with an existential type.

[GHC]

#15552: Infinite loop/panic with an existential type.
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        Reporter:  howtonotwin       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeInType,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14723            |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * keywords:   => TypeInType, TypeFamilies


Comment:

 The `-ddump-tc-trace` panic is an old GHC 8.4.3 bug that has since been
 fixed. (I can't recall if there was ever a ticket to track this.)

 I wasn't able to reproduce the infinite loop until I added some more
 language extensions (`TypeInType` and `TypeFamilies`):

 {{{#!hs
 {-# LANGUAGE DataKinds, ExistentialQuantification, GADTs, PolyKinds,
 TypeOperators #-}
 {-# LANGUAGE TypeInType, TypeFamilies #-}
 module T where

 import Data.Kind

 data Elem :: k -> [k] -> Type where
   Here :: Elem x (x : xs)
   There :: Elem x xs -> Elem x (y : xs)

 data EntryOfVal (v :: Type) (kvs :: [Type]) = forall (k :: Type).
 EntryOfVal (Elem (k, v) kvs)

 type family EntryOfValKey (eov :: EntryOfVal v kvs) :: Type where
   EntryOfValKey ('EntryOfVal (_ :: Elem (k, v) kvs)) = k
 type family GetEntryOfVal (eov :: EntryOfVal v kvs) :: Elem (EntryOfValKey
 eov, v) kvs where
   GetEntryOfVal ('EntryOfVal e) = e

 type family FirstEntryOfVal (v :: Type) (kvs :: [Type]) :: EntryOfVal v
 kvs where
   FirstEntryOfVal v ((k, v) : _) = 'EntryOfVal Here
   FirstEntryOfVal v (_ : kvs) = 'EntryOfVal (There (GetEntryOfVal
 (FirstEntryOfVal v kvs)))
 }}}

 This still loops when compiled even on 8.6 and HEAD. I wonder if there is
 any relationship between this ticket and #15473...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15552#comment:1>
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