[GHC] #12104: Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell into a hole"

[GHC]

#12104: Type families, `TypeError`, and `-fdefer-type-errors` cause "opt_univ fell
into a hole"
-------------------------------------+-------------------------------------
           Reporter:  antalsz        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  TypeFamilies,  |  Operating System:  MacOS X
  CustomTypeErrors                   |
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If I create a type family – open or closed – with a case that evaluates to
 a `TypeError`, and define a top-level binding with this type, loading the
 file with `-fdefer-type-errors` enabled (or via `:load!`/`:reload!`)
 panics GHC with "opt_univ fell into a hole".  (And if I used `:load!` or
 `:reload!`, `-fdefer-type-errors` doesn't get unset.)

 A minimal example:

 {{{#!hs
 {-# LANGUAGE TypeFamilies, DataKinds, UndecidableInstances #-}

 import GHC.TypeLits

 type family F a where
   F a = TypeError (Text "error")

 err :: F ()
 err = ()
 }}}

 results in the panic

 {{{
 ….hs:9:7: warning: [-Wdeferred-type-errors]
     • error
     • In the expression: ()
       In an equation for ‘err’: err = ()
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         opt_univ fell into a hole {a4Va}
 }}}

 Adding more cases to the type family, or making it open, still cause the
 crash.  This holds whether the error case is a final catch-all case, or
 something more like

 {{{#!hs
 type family F a where
   F () = TypeError (Text "error")
   F a  = ()
 }}}

 Just using a type synonym for `F` doesn't cause a panic, however, and nor
 does giving `err` the type `TypeError (Text "error")` directly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12104>
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