Re: [GHC] #12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into an infinite loop

6 Aug 2017

      #12860: GeneralizedNewtypeDeriving + MultiParamTypeClasses sends typechecker into
an infinite loop
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  FunDeps,
      Resolution:                    |  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The problem isn't limited to just derived instances. If you try to write
 out the code that would be derived by hand:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Bug where

 class C a b | a -> b where
   c :: a -> Int

 newtype Foo a = Foo a

 instance C b a => C b (Foo a) where
   c (Foo a) = c a
 }}}

 Then GHC will also "loop" (i.e., it will stack overflow on GHC 8.0.2 or
 later, and properly loop on GHC 8.0.1):

 {{{
     • Reduction stack overflow; size = 201
       When simplifying the following type: C a0 (Foo a)
       Use -freduction-depth=0 to disable this check
       (any upper bound you could choose might fail unpredictably with
        minor updates to GHC, so disabling the check is recommended if
        you're sure that type checking should terminate)
     • In the expression: c a
       In an equation for ‘c’: c (Foo a) = c a
       In the instance declaration for ‘C b (Foo a)’
    |
 14 |   c (Foo a) = c a
    |               ^^^
 }}}

 So if anything, we should be placing an extra check in the constraint
 solver, not just GND.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12860#comment:11
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