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

[GHC]

#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):

 Well, maybe. The original program and the program in comment:11 have
 another key difference. The latter uses `UndecidableInstances`, so I
 suppose it's entirely reasonable that we get a stack overflow error on
 that one. The former program, on the other hand, does not use
 `UndecidableInstances`.

 Really, we ought to reject the former program outright with an error
 message similar to the one that we get if we leave off
 `UndecidableInstances` from the latter program:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 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
 }}}
 {{{
 Bug.hs:12:10: error:
     • Illegal instance declaration for ‘C b (Foo a)’
         The coverage condition fails in class ‘C’
           for functional dependency: ‘a -> b’
         Reason: lhs type ‘b’ does not determine rhs type ‘Foo a’
         Un-determined variable: a
         Using UndecidableInstances might help
     • In the instance declaration for ‘C b (Foo a)’
    |
 12 | instance C b a => C b (Foo a) where
    |          ^^^^^^^^^^^^^^^^^^^^
 }}}

 If we did this, then I would consider this issue resolved.

 This coverage condition check is performed in `checkValidInstance`.
 Curiously, we don't call `checkValidInstance` on derived instances at the
 comment. There are relevant comments
 [http://git.haskell.org/ghc.git/blob/b14c03737574895718eed786a60dfdfd42ab49ce:/compiler/typecheck/TcDerivInfer.hs#l594
 here]:

 {{{#!hs
                 -- checkValidInstance tyvars theta clas inst_tys
                 -- Not necessary; see Note [Exotic derived instance
 contexts]

            ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
                 -- Claim: the result instance declaration is guaranteed
 valid
                 -- Hence no need to call:
                 --   checkValidInstance tyvars theta clas inst_tys
 }}}

 As this ticket shows, this isn't quite right. So perhaps we need to carve
 out the bit that checks the coverage condition and just check that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12860#comment:12>
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