help understanding a ghc assert failure wrt coercions

Carter Schonwald carter.schonwald at
Mon Jan 6 03:11:30 UTC 2020

Hey all,
I've been spending a little bit of time hacking out my own little "erase
branch, and with asserts enabled i'm seeing a very strange panic that seems
to be invalid?
namely: its stating that the types have changed,

BUT the error output indicates that the  types  are the same on both sides!

ghc: panic! (the 'impossible' happened)
  (GHC version
ASSERT failed!
  optCoercion changed types!
    Univ(representational Erased
         :: IO a_a2Pd, State# RealWorld -> (# State# RealWorld, a_a2Pd #))
  in_ty1: IO a_a2Pd
  in_ty2: State# RealWorld -> (# State# RealWorld, a_a2Pd #)
    Univ(representational Erased
         :: IO a_a2Pd, State# RealWorld -> (# State# RealWorld, a_a2Pd #))
  out_ty1: IO a_a2Pd
  out_ty2: State# RealWorld -> (# State# RealWorld, a_a2Pd #)
       In scope: InScope {wild_00 augment ++ build foldr eqString bindIO
                          returnIO otherwise assert thenIO breakpoint
breakpointCond map $
                          join f_a19W x_a19X x_a1hI s_a1hZ $c==_a2rD
$c/=_a2rM $cp1Ord_a2rY
                          $ccompare_a2s0 $c<_a2se $c<=_a2ss $c>_a2sy
$c>=_a2sE $cmax_a2sL
                          $cmin_a2sS $cp1Monoid_a2t4 $cmappend_a2tf
$cmconcat_a2tl $c>>=_a2tD
                          $c>>_a2tS $creturn_a2u3 $cpure_a2uo $c<*>_a2uA
                          $c*>_a2v0 $c<*_a2vb $c>>=_a2vw $c>>_a2vJ
$creturn_a2vU $cpure_a2wd
                          $c<*>_a2wn $cliftA2_a2wy $c*>_a2wL $c<*_a2wW
$c>>=_a2xf $c>>_a2xq
                          $creturn_a2xB $c<*>_a2y0 $cliftA2_a2y9 $c*>_a2yj
                          $cmappend_a2yS $cmconcat_a2yY $cmappend_a2ze
                          $cp1Monoid_a2zE $cmempty_a2zG $cmappend_a2zU
                          $cp1Monoid_a2Ai $cmempty_a2Ak $cmappend_a2Aw
                          $cp1Monoid_a2AS $cmempty_a2AU $cmappend_a2B4
                          $cp1Monoid_a2Bo $cmempty_a2Bq $cmappend_a2By
                          $cmappend_a2BU $cmconcat_a2C0 $cp1Monoid_a2Cb
                          $cmconcat_a2Cp $cmappend_a2CH $cmconcat_a2CN
                          $csconcat_a2D8 $cstimes_a2De $c<>_a2Dt
$csconcat_a2DA $c<>_a2DV
                          $csconcat_a2DZ $cstimes_a2E5 $c<>_a2Eu
$csconcat_a2EI $cstimes_a2EO
                          $c<>_a2Fp $csconcat_a2FB $cstimes_a2FH $c<>_a2Gc
                          $cstimes_a2Gs $c<>_a2GR $csconcat_a2GZ
$cstimes_a2H5 $c<>_a2Hm
                          $csconcat_a2Hq $cstimes_a2Hu $c<>_a2HG
$csconcat_a2HM $cstimes_a2HS
                          $c<>_a2I6 $csconcat_a2Id $cstimes_a2Ij
$csconcat_a2IC $cstimes_a2II
                          $c>>=_a2J0 $c>>_a2JO $creturn_a2JZ $cpure_a2Ke
                          $cliftA2_a2Kw $c*>_a2KJ $c<*_a2KU $cfmap_a2L7
                          $cmzero_a2LF $cmplus_a2LO $cmzero_a2M7
$cmplus_a2Mg $cmzero_a2Mz
                          $cmplus_a2MI $csome_a2Na $cmany_a2Nj $csome_a2NK
                          $c<|>_a2Oe $csome_a2Oj $cmany_a2Os $c>>_a2OP
                          $cfmap_a2Pb $c<$_a2Ps a_a2Pu b_a2Pv $c>>=_a2PJ
                          $c>>=_a2Qh $creturn_a2Qz $c>>=_a2QP $c>>_a2QV
                          $c<*>_a2Rr $cliftA2_a2RC $c<*_a2RX $cpure_a2Se
                          $cliftA2_a2Sr $c*>_a2SA $c<*_a2SI $c<*>_a2T5
                          $c*>_a2Tq $c<*_a2Tx $c<*>_a2TW $cliftA2_a2U2
$c*>_a2U9 $c<*_a2Uk
                          $c<$_a2UF $cfmap_a2US $c<$_a2V0 $cfmap_a2Ve
$c<$_a2Vk $c<$_a2VH
                          $cfmap_a2VX $c<$_a2W3 $cfmap_a2Wi $c<$_a2Wo
$krep_a37a $krep_a37b
                          $krep_a37c $krep_a37d $krep_a37e $krep_a37f
$krep_a37g $krep_a37h
                          $krep_a37i $krep_a37j $krep_a37k $krep_a37l
$krep_a37m $krep_a37n
                          $krep_a37o $krep_a37p $krep_a37q $krep_a37r
$krep_a37s $sap_d3dm
                          $sap_d3dn $sliftM5_d3ds $sliftM5_d3dt
$sliftM4_d3dy $sliftM4_d3dz
                          $sliftM3_d3dE $sliftM3_d3dF $sliftM2_d3dK
                          $sliftM_d3dQ $sliftM_d3dR $swhen_d3e3 $swhen_d3e4
                          $sliftA3_d3eh $sliftA3_d3ei $sliftA_d3em
$sliftA_d3en $tcMonoid
                          $tcSemigroup $trModule <**> liftA liftA3 =<< when
sequence mapM
                          liftM liftM2 liftM3 liftM4 liftM5 ap mapFB
unsafeChr ord minInt
                          maxInt id const . flip $! until asTypeOf failIO
unIO getTag quotInt
                          remInt divInt modInt quotRemInt divModInt
divModInt# shiftL#
                          shiftRL# iShiftL# iShiftRA# iShiftRL# $tcFunctor
$dm<$ $fFunctor[]
                          $fFunctorMaybe $fFunctor(,) $fFunctor->
                          $fFunctor(,,) $tcApplicative $dm<*> $dmliftA2
$dm*> $dm<*
                          $fApplicativeIO $fApplicative[]
$fApplicativeMaybe $fApplicative->
                          $tcMonad $dm>> $dmreturn $fMonadIO $fFunctorIO
                          $fMonadMaybe $fMonad-> $tcAlternative $dmsome
                          $fAlternativeIO $fAlternative[]
$fAlternativeMaybe $tcMonadPlus
                          $dmmzero $dmmplus $fMonadPlusIO $fMonadPlus[]
                          $tc':| $tcNonEmpty $fMonadNonEmpty
                          $fFunctorNonEmpty $dmsconcat $dmstimes
                          $fSemigroupMaybe $fSemigroupOrdering
                          $fSemigroup(,,,) $fSemigroup(,,) $fSemigroup(,)
                          $fSemigroup-> $fSemigroupNonEmpty $fSemigroup[]
                          $dmmconcat $tc'C:Monoid $fMonoidIO $fMonad(,,,)
                          $fMonad(,,) $fApplicative(,,) $fMonad(,)
                          $fMonoidMaybe $fMonoidOrdering $fMonoid(,,,,)
                          $fMonoid(,,) $fMonoid(,) $fMonoid() $fMonoid->
$fMonoid[] $tc'O
                          $tcOpaque $fEqNonEmpty $fOrdNonEmpty
returnIO_s3A8 failIO_s3A9
                          $cempty_s3Ab unIO_s3Ac thenIO_s3Ah bindIO_s3Am
                          $trModule_s3AM $trModule_s3AN $trModule_s3AO
                          $tcFunctor_s3AQ $tcFunctor_s3AR
                          $tcApplicative_s3AT $tcMonad_s3AU $tcMonad_s3AV
                          $tcAlternative_s3AX $tcMonadPlus_s3AY
                          $tcNonEmpty_s3B0 $tcNonEmpty_s3B1 $krep_s3B2
                          $tc':|_s3B4 $tcSemigroup_s3B5 $tcSemigroup_s3B6
                          $tcMonoid_s3B8 $tcMonoid_s3B9 $krep_s3Ba
                          $tc'C:Monoid_s3Bc $tcOpaque_s3Bd $tcOpaque_s3Be
                          $tc'O_s3Bg $cliftA2_s3BL $cfmap_s3BP $c<*>_s3BT
                          $sliftM2_s3BZ $sliftM5_s3Cc $sliftM4_s3Ch
                          $sliftM_s3Co f_s3FS}
       Type env: [a2Pd :-> b_a2Pv, a2Pe :-> a_a2Pu]
       Co env: []]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1173:37 in
        pprPanic, called at compiler/utils/Outputable.hs:1243:5 in
        assertPprPanic, called at compiler/types/OptCoercion.hs:122:41 in
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