[GHC] #15380: Infinite typechecker loop in GHC 8.6

[GHC]

#15380: Infinite typechecker loop in GHC 8.6
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following program loops infinitely during typechecking with GHC 8.6.1
 and HEAD:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind

 class Generic a where
   type Rep a :: Type

 class PGeneric a where
   type To a (x :: Rep a) :: a

 type family MDefault (x :: a) :: a where
   MDefault x = To (M x)

 class C a where
   type M (x :: a) :: a
   type M (x :: a) = MDefault x
 }}}

 In GHC 8.4.3, however this fails with a proper error:

 {{{
 $ /opt/ghc/8.4.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:15:16: error:
     • Occurs check: cannot construct the infinite kind:
         a ~ Rep (M x) -> M x
     • In the type ‘To (M x)’
       In the type family declaration for ‘MDefault’
     • Type variable kinds: x :: a
    |
 15 |   MDefault x = To (M x)
    |                ^^^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15380>
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