[GHC] #14332: Deriving clauses can have forall types

[GHC]

#14332: Deriving clauses can have forall types
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 OK, so to review:

 * We need to write up
 [https://ghc.haskell.org/trac/ghc/ticket/14331#comment:8 the
 rationalisation] in the user manual.

 * If we are going to make it a proper feature (which it already is), we
 should not require silly parens.  E.g. this example from the Description
 {{{
 data D a = D deriving ((forall a. C a))
 }}}
   works just as we intend, but should not require the second pair of
 parens -- a parser bug.

 * The second example from the Description is also OK
 {{{
 data Proxy (a :: k) = Proxy
   deriving ((forall k2. (Generic1 :: (k2 -> Type) -> Constraint)))
 }}}
   As I put it in the rationalisation, `k` comes from the data type decl,
 and `k2` from the instance; but it's fine to instantiate `k` to `k2` in
 that instance decl.  And that is just what happens.  Worth an example in
 the user manual perhaps, after the one about `Functor Proxy`.

 Ryan, might you be able to do this?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14332#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler