[GHC] #14873: GHC HEAD regression (piResultTy)

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[GHC] #14873: GHC HEAD regression (piResultTy)

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#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.5
  (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:
-------------------------------------+-------------------------------------
 (Originally noticed [https://travis-
 ci.org/goldfirere/singletons/jobs/347945148#L1179 here].)

 The following program typechecks on GHC 8.2.2 on GHC 8.4.1, but panics on
 GHC HEAD:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 {-# LANGUAGE TypeOperators #-}
 module Bug where

 import Data.Kind (Type)

 data family Sing (a :: k)

 newtype instance Sing (f :: k1 ~> k2) =
   SLambda { applySing :: forall t. Sing t -> Sing (Apply f t) }

 data TyFun :: Type -> Type -> Type
 type a ~> b = TyFun a b -> Type
 infixr 0 ~>
 type family Apply (f :: k1 ~> k2) (x :: k1) :: k2

 class SingI (a :: k) where
   sing :: Sing a

 data ColSym1 :: f a -> a ~> Bool
 type instance Apply (ColSym1 x) y = Col x y

 class PColumn (f :: Type -> Type) where
   type Col (x :: f a) (y :: a) :: Bool

 class SColumn (f :: Type -> Type) where
   sCol :: forall (x :: f a) (y :: a).
     Sing x -> Sing y -> Sing (Col x y :: Bool)

 instance (SColumn f, SingI x) => SingI (ColSym1 (x :: f a) :: a ~> Bool)
 where
   sing = SLambda (sCol (sing @_ @x))
 }}}

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.5.20180201 for x86_64-unknown-linux):
         piResultTy
   k_aZU[tau:1]
   (a_aW8[sk:1] |> <*>_N)
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in
 ghc:Outputable
         pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * cc: simonpj (added)


Comment:

 This regression was introduced in commit
 0a12d92a8f65d374f9317af2759af2b46267ad5c (`Further improvements to well-
 kinded types`).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:1>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Urk!  Patch coming -- but validation is not complete and I have to go
 home, so probably Monday.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:2>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"3d252037234ce48f9bdada7d5c9b1d8eba470829/ghc"
 3d252037/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3d252037234ce48f9bdada7d5c9b1d8eba470829"
 Respect Note [The tcType invariant]

 I tried to do this with

     commit 0a12d92a8f65d374f9317af2759af2b46267ad5c
     Author: Simon Peyton Jones <[hidden email]>
     Date:   Wed Dec 13 12:53:26 2017 +0000

     Further improvements to well-kinded types

     The typechecker has the invariant that every type should be well-
 kinded
     as it stands, without zonking.  See Note [The well-kinded type
 invariant]
     in TcType.

     That invariant was not being upheld, which led to Trac #14174.  I
 fixed
     part of it, but T14174a showed that there was more.  This patch
 finishes
     the job.

 But I didn't get it quite right as Trac #14873 showed.

 This patch fixes the problem; although I am still a bit unhappy.
 (See "A worry" in the HsApp case of tc_infer_hs_type.)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:3>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Changes (by simonpj):

 * owner:  (none) => goldfire


Comment:

 OK done.  Richard: could you have a look, to check my work?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:4>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

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In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * keywords:   => TypeInType


--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:5>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 I think we need these invariants:

 1. If `ty <- tc_hs_type ... exp_kind`, then `typeKind ty == exp_kind`.

 2. If `(ty, ki) <- tc_infer_hs_type ...`, then `typeKind ty == ki`.

 3. If `(ty, ki) <- tc_infer_hs_type ...`, then `zonk ki == ki`.

 4. If `(ty, ki) <- tcTyVar ...`, then `typeKind ty == ki`.

 5. If `(ty, ki) <- tcTyVar ...`, then `zonk ki == ki`.

 All of these appear to be true now, except I'm worried about the `tcTyVar`
 case for `TcTyCon`s -- I ''think'' `TcTyCon` kinds can have unzonked
 metavariables, if the `TcTyCon` comes from the non-CUSK case in
 `kcLHsQTyVars`. Do you also think this is possible? If so, we need to zonk
 the kind and wrap the (knot-tied) type in a reflexive coercion (with
 `mkNakedCastTy`) to fix its kind. Do you agree?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:6>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

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In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * milestone:   => 8.6.1


--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:7>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by Richard Eisenberg <rae@…>):

 In [changeset:"cf67e59a90bcaba657a9f5db3d5defb6289c274f/ghc"
 cf67e59a/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="cf67e59a90bcaba657a9f5db3d5defb6289c274f"
 Expand and implement Note [The tcType invariant]

 Read that note -- it's necessary to make sure that we can
 always call typeKind without panicking. As discussed on #14873,
 there were more checks and zonking to do, implemented here.
 There are no known bugs fixed by this patch, but there are likely
 unknown ones.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:8>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

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In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------
Changes (by goldfire):

 * status:  new => merge


Comment:

 This ticket just had some documentation needs to sort out, but along the
 way, I found places where a few zonks were necessary. I see no reason not
 to merge.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:9>
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Re: [GHC] #14873: GHC HEAD regression (piResultTy)

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In reply to this post by GHC - devs mailing list
#14873: GHC HEAD regression (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Richard, I'm beginning to wonder if the pain of `Note [The well-kinded
 type invariant]` is worth the gain.  It's all very delicate.

 Here's a possible alternative

 1. Define `tcTypeKind :: TcType -> TcM TcKind` which zonks, if necessary,
 on the fly.  This would be dead easy to write.

 2. Call `tcTypeKind` instead of `typeKind` in ... well, in various places.
 Anywhere in the type checker where un-zonked kinds might be floating
 about.

 For (2) there are two difficulties

 * Which calls, exactly, would need to be in the monad?
 * Are there any calls that don't have the `TcM` monad conveniently to
 hand?  For example `eqType` calls `typeKind`, so we'd really need a
 `tcEqType` variant that calls `tcTypeKind`.

 Advantages: (a) redundant Refls disappear much earlier; this is Good (b)
 less delicate invariants.

 What do you think?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:10>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType) (was: GHC HEAD regression (piResultTy))

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#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:11>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"e24da5edb4709bdb050c8d0676f302d0b87b8446/ghc" e24da5e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e24da5edb4709bdb050c8d0676f302d0b87b8446"
 Better Note [The well-kinded type invariant]

 c.f. Trac #14873
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:12>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 I've wondered about this, too. If we had `TcType` separate from `Type`,
 then we this would be very easy to find.

 I suppose we could smoke it out: put an `ASSERT` in the `typeKind` case
 for `TyVarTy` that the tyvar isn't a `TcTyVar`. Do the same with `eqType`.
 Then we'd catch (almost) all the cases.

 I wonder what the performance implications would be.

 (+) We don't have to zonk unnecessarily just to maintain the invariant.

 (-) When we do zonk, we don't memorialize this in the type -- only in the
 kind.

 (-) Is monadic code slower than pure code?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:13>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Another thing that came up yesterday: there are some calls to `eqType`
 where we know for sure the the kinds are the same, so checking the kinds
 as well is redundant.  e.g. the use of `eqType` in `checkExpectedKinds`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:14>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * owner:  goldfire => (none)
 * status:  merge => new


Comment:

 comment:8 merged in 634c07dc2bd9b2be53d707d613df9e7100d543aa.

 Leaving open due to comment:14.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:15>
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Re: [GHC] #14873: The well-kinded type invariant (in TcType)

GHC - devs mailing list
In reply to this post by GHC - devs mailing list
#14873: The well-kinded type invariant (in TcType)
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
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:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * milestone:  8.6.1 => 8.8.1


--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:16>
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