Type holes cause assertion failure in ghc-stage2 compiler during type checking

ghc-stage2 panic! due to assertion failure when compiling the following code with ghc-stage2 Bug.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:

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.

Trac metadata

Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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