[GHC] #12921: initTc: unsolved constraints

[GHC]

#12921: initTc: unsolved constraints
-------------------------------------+-------------------------------------
           Reporter:  jlp            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  MacOS X
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 While working on a small project of mine, GHC panicked with the following
 error:
 {{{
 lua-types-0.1.0.0: build
 Preprocessing executable 'lua-types' for lua-types-0.1.0.0...
 [2 of 3] Compiling Lua.Parser       ( src/Lua/Parser.hs, .stack-
 work/dist/x86_64-osx/Cabal-1.24.1.0/build/lua-types/lua-types-
 tmp/Lua/Parser.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1.20161117 for x86_64-apple-darwin):
         initTc: unsolved constraints
   WC {wc_insol =
         [W] explist_a3nf :: t_a3ne[tau:1] (CHoleCan: explist)
         [W] funccall_a4dv :: t_a4du[tau:1] (CHoleCan: funccall)}

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

 From a quick google search, it seems like this is a bug in the type
 checker. I'm not willing nor able to dive any further into the GHC code
 myself, though.

 I'll attach the code as it is, before looking for a workaround. I'm using
 stack, GHC version 8.0.1.20161117 for x86_64-apple-darwin.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12921>
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