[GHC] #15678: Provide the provenance of unification variables in error messages when possible

[GHC]

#15678: Provide the provenance of unification variables in error messages when
possible
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 I think this would be great.

 One small extra piece.  When unifying `alpha ~ beta` we could be careful
 to eliminate the one with the least-informative provenance, if the
 unification can be solved either way round.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15678#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler