[GHC] #15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into a hole' panic

[GHC]

#15368: Type families, holes and -fdefer-type-errors may cause 'opt_univ fell into
a hole' panic
-------------------------------------+-------------------------------------
           Reporter:  dramforever    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This program, when compiled like `ghc -fdefer-type-errors -c panic.hs`:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}

 module MorePanic where

 transitive :: (a, b) -> (b, c) -> (a, c)
 transitive = undefined

 trigger :: a -> b -> (F a b, F b a)
 trigger _ _ = _ `transitive` trigger _ _

 type family F (n :: *) (m :: *) :: *
 }}}

 Gives this panic after several error messages:

 {{{
 ghc.EXE: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-mingw32):
         opt_univ fell into a hole
   {co_a7Sj}
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler\utils\Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler\\types\\OptCoercion.hs:242:5 in
 ghc:OptCoercion

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 This bug also affects GHCi. Attached is full output

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15368>
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