Re: [GHC] #15918: mkCastTy sometimes drops insoluble (Type ~ Constraint) coercions

7 Mar 2019

      #15918: mkCastTy sometimes drops insoluble (Type ~ Constraint) coercions
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  quantified-
                                     |  constraints/T15918
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/489
-------------------------------------+-------------------------------------
Description changed by simonpj:

Old description:
...
'''EDIT: See comment:3 for the real cause and comment:17 for the current
plan.'''
See also:
* TyCoRep `Note [Respecting definitional equality]` and its supporting
`Note [Non-trivial definitional equality]`
* #11715 Constraint vs *
* #15799
TL;DR: there's a bug in here, that we don't know how to solve.  But it's
not biting us, and once we solve #11715 that may point the way.
---------------------------
Minimized from
https://gist.github.com/Icelandjack/683bd4b79027695ffc31632645c9d58b, I
don't expect `Build []` to kind check but ti shouldn't crash.
{{{#!hs
{-# Language PolyKinds             #-}
{-# Language TypeFamilies          #-}
{-# Language ConstraintKinds       #-}
{-# Language FlexibleContexts      #-}
{-# Language QuantifiedConstraints #-}
{-# Language UndecidableInstances  #-}
import Data.Kind
class Rev f where
  rev :: f a
instance (forall xx. cls xx => Rev xx) => Rev (Build cls) where
  rev = undefined
data Build :: ((k -> Type) -> Constraint) -> (k -> Type)
uu = rev :: Build [] a
}}}
gives a panic
{{{
$ ./ghc-stage2 --interactive -ignore-dot-ghci ~/hs/711_bug.hs
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( ~/hs/711_bug.hs, interpreted )
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.7.20181029 for x86_64-unknown-linux):
        ASSERT failed!
  irred_a1zW :: [(xx_a1zV[sk:3] |> Sym {co_a1zu})]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
ghc:Outputable
        assertPprPanic, called at compiler/typecheck/TcType.hs:1826:53 in
ghc:TcType
Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
...
}}}
New description:

 '''EDIT: See comment:3 for the real cause and comment:17 for the current
 plan.'''

 See also:
 * TyCoRep `Note [Respecting definitional equality]` and its supporting
 `Note [Non-trivial definitional equality]`
 * #11715 Constraint vs *
 * #15799
 * #13650

 TL;DR: there's a bug in here, that we don't know how to solve.  But it's
 not biting us, and once we solve #11715 that may point the way.

 ---------------------------

 Minimized from
 https://gist.github.com/Icelandjack/683bd4b79027695ffc31632645c9d58b, I
 don't expect `Build []` to kind check but ti shouldn't crash.

 {{{#!hs
 {-# Language PolyKinds             #-}
 {-# Language TypeFamilies          #-}
 {-# Language ConstraintKinds       #-}
 {-# Language FlexibleContexts      #-}
 {-# Language QuantifiedConstraints #-}
 {-# Language UndecidableInstances  #-}

 import Data.Kind

 class Rev f where
   rev :: f a

 instance (forall xx. cls xx => Rev xx) => Rev (Build cls) where
   rev = undefined

 data Build :: ((k -> Type) -> Constraint) -> (k -> Type)

 uu = rev :: Build [] a
 }}}

 gives a panic

 {{{
 $ ./ghc-stage2 --interactive -ignore-dot-ghci ~/hs/711_bug.hs
 GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( ~/hs/711_bug.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.7.20181029 for x86_64-unknown-linux):
         ASSERT failed!
   irred_a1zW :: [(xx_a1zV[sk:3] |> Sym {co_a1zu})]
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in
 ghc:Outputable
         pprPanic, called at compiler/utils/Outputable.hs:1219:5 in
 ghc:Outputable
         assertPprPanic, called at compiler/typecheck/TcType.hs:1826:53 in
 ghc:TcType

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

--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15918#comment:20
GHC http://www.haskell.org/ghc/
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