[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Iceland_jack):

 Possibly related, should this complain or is it up to the caller to use it
 with a `Monoid A` instance in scope

 {{{#!hs
 import Control.Monad.Writer

 data A = A

 a :: MonadWriter A m => m ()
 a = tell A
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:29>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler