[GHC] #12677: Type equality in constraint not used?

[GHC]

#12677: Type equality in constraint not used?
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 This is a hard one to fix, surprisingly. The problem is that you've
 declared `a` to have type `k` and then used `a` as an argument to `->`. Of
 course, `->` expects a `Type`. So GHC should insert add a cast, forming `a
 |> η`, for the argument to `->`. The problem is that this would require
 using the equality proof for `Type ~ k` as the body of `η`, and the
 current type system simply does not allow this.
 [https://ghc.haskell.org/trac/ghc/wiki/DependentHaskell/Internal#Liftedvs.Unliftedequality
 Here] are some notes on the matter, though they were not written to be
 digested by anyone but me.

 I don't expect this will be fixed until we have `-XDependentTypes`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12677#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler