[GHC] #15450: Inconsistency w.r.t. coverage checking warnings for EmptyCase under unsatisfiable constraints

[GHC]

#15450: Inconsistency w.r.t. coverage checking warnings for EmptyCase under
unsatisfiable constraints
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12957            |  Differential Rev(s):  Phab:D5017
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 > This is all explained in the commentary in
 ​https://phabricator.haskell.org/D3064 (which I've attempted to turn into
 Note form in this patch).

 Thank you for turning it into a Note.

 Yes, I am questioning the principle. It seems bizarrely discontinuous to
 fling the door wide open when (but only when) it completely closes.
 Indeed I see that on Phab:D3064 George did suggest "The patch as is just
 drops information, we can instead issue no warnings if we are already in a
 dead-code branch."

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15450#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler