[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 goldfire):

 I think comment:4 is unrelated. (But let's not lose the fact that there is
 a terrible pretty-printing bug there, without parentheses around the
 equality argument to `Dict`.) The commit in comment:7 is all about
 instances, and comment:4 is unrelated to instances.

 Yes, I think the original code should produce a warning. To suppress the
 warning, the user should include an implementation of `foo`, so that the
 generic-default implementation (which has the wrong type) is not used. But
 if inaccessible code is an error, then the original code should produce an
 error, just as it is now.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler