[GHC] #4528: stand-alone deriving sometimes fails for GADTs

[GHC]

#4528: stand-alone deriving sometimes fails for GADTs
------------------------------------------+---------------------------------
    Reporter:  nestra                     |        Owner:                              
        Type:  bug                        |       Status:  new                         
    Priority:  normal                     |    Milestone:                              
   Component:  Compiler (Type checker)    |      Version:  7.0.1                       
    Keywords:  deriving mechanism, GADTs  |     Testcase:                              
   Blockedby:                             |   Difficulty:                              
          Os:  Linux                      |     Blocking:                              
Architecture:  x86_64 (amd64)             |      Failure:  Compile-time performance bug
------------------------------------------+---------------------------------

Comment(by simonpj):

 Thanks for pointing this out.

 It's not reasonable to derive an `Enum` instance for a GADT.  Consider
 {{{
     data T a where
       T1 :: T Int
       T2 :: T Bool
       T3 :: T a
 }}}
 What would `[T1 ..`] be?  Clearly not `[T1,T2,T3]` because that would be
 ill-typed.  Perhaps `[T1,T3] :: T Int`?  It is very far from clear.  I
 have added a check to treat GADTs as non-enumerations.

 The same holds for `Bounded`.  After, all `minBound` for `T` should have
 type
 {{{
 minBoundT :: forall a. T a
 }}}
 but the `minBound` is `T1`, which doesn't have that type.

 For `Read` things are less clear.  In general the "standalone deriving"
 mechanism generates the boilerplate code and typechecks it; if it doesn't
 typecheck you get a complaint (see #3012).  So the error messages for
 `Read` seem right.  Situation is different for `Enum` because it uses the
 primop `tagToEnum#` which must be treated with care.

 I'll commit a fix shortly.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4528#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler