[GHC] #15546: Display coaxiom branch incompatibilities in GHCi

[GHC]

#15546: Display coaxiom branch incompatibilities in GHCi
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           Reporter:  mniip          |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  GHCi           |           Version:  8.4.3
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  GHCi                               |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Currently the only way to figure out how GHC figured out compatibility of
 branches in a closed type family is to compile it to `.hi` and then sieve
 through `--show-iface` output.

 Something as simple this should suffice:

 {{{#!hs
 > :i ==
 type family (==) (a :: k) (b :: k) :: Bool
   where
       (==) (f a) (g b) = (f == g) && (a == b)
       (==) a a = 'True    -- incompatible with: 0
       (==) _1 _2 = 'False -- incompatible with: 0, 1
         -- Defined in ‘Data.Type.Equality’
 infix 4 ==
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15546>
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