[GHC] #13258: GHC head 8.1.20170209 does not compile package 'mono-traversable'

[GHC]

#13258: GHC head 8.1.20170209 does not compile package 'mono-traversable'
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13249            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => closed
 * resolution:   => invalid
 * related:   => #13249


Comment:

 This is expected behavior.

 GHC 8.2 tightened up validity checking for default type signatures. This
 catches many things which are just plain type incorrect (e.g., see
 #12918). A consequence of this is that you can't write default type
 signatures quite as liberally as before, but luckily, it's an extremely
 easy fix:

 {{{#!hs
 class MonoFunctor mono where
     omap :: (Element mono -> Element mono) -> mono -> mono
     default omap :: (Functor f, Element (f a) ~ a, f a ~ mono)
                  => (Element mono -> Element mono) -> mono -> mono
     omap = fmap
     {-# INLINE omap #-}
 }}}

 The benefit is now the connection between the default and non-default type
 signatures is much clearer. That is, the default type signature is exactly
 the same as the non-default one, just with more constraints.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13258#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler