[GHC] #8883: FlexibleContexts checking should happen also on inferred signature

[GHC]

#8883: FlexibleContexts checking should happen also on inferred signature
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        Reporter:  Blaisorblade      |            Owner:  jstolarek
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by jstolarek):

 Replying to [comment:1 simonpj]:
 > if someone would like to have a go it would not be hard and I could
 offer guidance.
 I'd like to tackle this one. Guidance will be really appreciated.

 I reduced the test case to this snippet of code:

 {{{
 {-# LANGUAGE TypeFamilies, DeriveFunctor, NoMonomorphismRestriction #-}
 -- , FlexibleContexts
 module T8883 where

 type family PF a :: * -> *

 class Regular a where
   from :: a -> PF a a

 -- But in fact, the construction is just an instance of fold.
 --fold :: (Functor (PF a), Regular a) => (PF a b -> b) -> a -> b
 fold f = f . fmap (fold f) . from
 }}}

 So how should GHC behave for this code? Reject it because
 !FlexibleContexts extension is not enabled?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8883#comment:5>
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