[GHC] #15289: isUnliftedType GHC panic on pattern with True :: Maybe

[GHC]

#15289: isUnliftedType GHC panic on pattern with True :: Maybe
-------------------------------------+-------------------------------------
           Reporter:  Remi           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:  aarch64        |   Type of failure:  Compile-time
                                     |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 module Oops where

 pattern What = True :: Maybe
 }}}

 Fails on 8.4.3 with:
 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.4.3 for x86_64-unknown-linux):
         isUnliftedType
   (Maybe |> {co_a1Dg3}) :: TYPE t_a1Dg9[tau:1]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:1939:10 in ghc:Type

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

 I'm currently cloning from git, and will try to get around to check it
 with HEAD over the next days. BTW, it's not a show-stopper for me.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15289>
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