[GHC] #13324: Allow PartialTypeSignatures in the instance context of a standalone deriving declaration

[GHC]

#13324: Allow PartialTypeSignatures in the instance context of a standalone
deriving declaration
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #10607            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 We should make sure to watch out for these two corner cases:

 * Exotic constraints. Currently, trying to derive `Eq` like this:

   {{{#!hs
   newtype Foo f a = Foo (f (f a)) deriving Eq
   }}}

   Will be rejected with an error message saying:

   {{{
     • No instance for (Eq (f (f a)))
         arising from the 'deriving' clause of a data type declaration
       Possible fix:
         use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
   }}}

   That is, GHC will not put `Eq (f (f a))` into the set of inferred
 constraints because it is too "exotic". But presumably, we want a similar
 failure if the user instead typed this:

   {{{#!hs
   newtype Foo f a = Foo (f (f a))
   deriving instance _ => Eq (Foo f a)
   }}}

   Since it's morally equivalent to just using a `deriving Eq` clause.
 * GADTs. GHC chooses not to even make an attempt if you try to stock-
 derive certain instances for GADTs with a `deriving` clause. For instance,
 this:

   {{{#!hs
   data T a where
       MkT :: T Int
     deriving Eq
   }}}

   is rejected with:

   {{{
     • Can't make a derived instance of ‘Eq (T a)’:
         Constructor ‘MkT’ has existentials or constraints in its type
         Possible fix: use a standalone deriving declaration instead
     • In the data declaration for ‘T’
   }}}

   So as a consequence, we should probably disallow this as well:

   {{{#!hs
   data T a where
       MkT :: T Int
   deriving instance _ => Eq (T a)
   }}}

 Notice that in both cases, the error message suggests trying a standalone
 deriving declaration! Obviously, we shouldn't do that if a user tries the
 equivalent, standalone approach with wildcards, so we should come up with
 a more appropriate error message for this scenario.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13324#comment:1>
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