[GHC] #8851: Standalone deriving and GND behave differently

[GHC]

#8851: Standalone deriving and GND behave differently
--------------------------------------------+------------------------------
        Reporter:  simonpj                  |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.8.1-rc1
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
--------------------------------------------+------------------------------

Comment (by nomeata):

 That’s my gut feeling as well. But what if the type checker resolves the
 constraints in `forall a. (Monad IdrisInnerParser, Show a) =>
 IdrisInnerParser a -> IdrisInnerParser ()’` before coerce is being called,
 so that it would only have to coerce `forall a. (Show a) =>
 IdrisInnerParser a -> IdrisInnerParser ()’, and that would be fine. This
 is probably what is happening in case (A) (otherwise the result would not
 role-type-check).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8851#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler