[GHC] #9235: Simplifier ticks exhausted on recursive class method

[GHC]

#9235: Simplifier ticks exhausted on recursive class method
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       Reporter:  klao             |             Owner:
           Type:  bug              |            Status:  new
       Priority:  normal           |         Milestone:
      Component:  Compiler         |           Version:  7.8.2
       Keywords:                   |  Operating System:  Unknown/Multiple
   Architecture:                   |   Type of failure:  Compile-time crash
  Unknown/Multiple                 |         Test Case:
     Difficulty:  Unknown          |          Blocking:
     Blocked By:                   |
Related Tickets:                   |
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 GHC 7.8 and HEAD panic on the following code:

 {{{#!haskell
 module C where

 class C a where
   c :: C a => a -> Int
   c a = c a
   {-# INLINE c #-}

 instance C ()

 x :: Int
 x = c ()
 }}}

 Note that it's the coincidence of the 3 conditions that triggers this bug:
 1. the `c` method has to be inlined (explicity or implicitly)
 2. it has to be recursive (but, it doesn't have to be as dumbly recursive
 as in the example)
 3. it has to have this weird, slightly mistaken type signature: `C a => a
 -> Int` instead of simply `a -> Int`.

 But, when this incidentally happens (in my case, both 2. ''and'' 3. were
 unintentional), the user gets a very hard to debug issue. They don't even
 get an indication which module is responsible if `c` is used from a
 different module than the one with the class definition.

 Also note that this happens even with a non-optimized build. And in this
 example `c` has to be explicitly marked NOINLINE to prevent it. (In the
 instance that I caught in the wild simply removing the INLINE annotation
 was enough.)

 Finally, on a side note: shouldn't 3. by itself be an error? Or a warning
 at least?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9235>
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