[GHC] #14884: Type holes cause assertion failure in ghc-stage2 compiler during type checking

3 Mar 2018

      #14884: Type holes cause assertion failure in ghc-stage2 compiler during type
checking
-------------------------------------+-------------------------------------
           Reporter:  sighingnow     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 ghc-stage2 panic! due to assertion failure when compiling the following
 code with `ghc-stage2 Bug.hs`

 {{{#!hs
 module Bug where

 x :: IO ()
 x = _ print "abc"
 }}}

 Callstack:

 {{{
 λ inplace\bin\ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20180225 for x86_64-unknown-mingw32):
         ASSERT failed!
   t_a4ec[tau:2]
   2
   1
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler\utils\Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler\utils\Outputable.hs:1206:5 in
 ghc:Outputable
         assertPprPanic, called at compiler\\typecheck\\TcType.hs:1187:83
 in ghc:TcType
 CallStack (from -prof):
   TcInteract.solve_loop
 (compiler\typecheck\TcInteract.hs:(247,9)-(254,44))
   TcInteract.solveSimples
 (compiler\typecheck\TcInteract.hs:(241,5)-(243,21))
   TcRnDriver.simplifyTop (compiler\typecheck\TcRnDriver.hs:408:25-39)
   TcRnDriver.tcRnSrcDecls (compiler\typecheck\TcRnDriver.hs:254:25-65)
 }}}

 The failed assertion is `checkTcLevelInvariant ctxt_tclvl tv_tclvl` in
 `isTouchableMetaTyVar`:

 {{{#!hs
 isTouchableMetaTyVar :: TcLevel -> TcTyVar -> Bool
 isTouchableMetaTyVar ctxt_tclvl tv
   | isTyVar tv -- See Note [Coercion variables in free variable lists]
   = ASSERT2( tcIsTcTyVar tv, ppr tv )
     case tcTyVarDetails tv of
       MetaTv { mtv_tclvl = tv_tclvl }
         -> ASSERT2( checkTcLevelInvariant ctxt_tclvl tv_tclvl,
                     ppr tv $$ ppr tv_tclvl $$ ppr ctxt_tclvl )
            tv_tclvl `sameDepthAs` ctxt_tclvl
       _ -> False
   | otherwise = False
 }}}

 Notice that the ghc-stage1 compiler doesn't panic and report the type hole
 correctly. This seems a regression and I have checked that ghc-8.2.2 also
 works well.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14884
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