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

[GHC]

#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>
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