[GHC] #11674: GHC accepts overly general instance sigs

[GHC]

#11674: GHC accepts overly general instance sigs
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:  InstanceSigs   |  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:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE InstanceSigs #-}

 data I a = MkI a

 instance Functor I where
   fmap :: (a -> b) -> f a -> f b
   fmap = undefined
 }}}

 is accepted when the type is too general, even things like:

 {{{#!hs
   fmap :: (a -> b) -> f a -> g b
 -- &
   fmap :: whoami
 }}}

 are accepted

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11674>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler