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  • #16225

Closed
Open
Opened Jan 23, 2019 by Ryan Scott@RyanGlScott
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GHC HEAD-only Core Lint error (Trans coercion mis-match)

The following code compiles on GHC 8.0.2 through GHC 8.6.3 with -dcore-lint:

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Bug where

import Data.Kind

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

data TyCon1 :: (k1 -> k2) -> (k1 ~> k2)
type instance Apply (TyCon1 f) x = f x

data SomeApply :: (k ~> Type) -> Type where
  SomeApply :: Apply f a -> SomeApply f

f :: SomeApply (TyCon1 Sing :: k ~> Type)
  -> SomeApply (TyCon1 Sing :: k ~> Type)
f (SomeApply s)
 = SomeApply s

However, it chokes on GHC HEAD:

$ ~/Software/ghc4/inplace/bin/ghc-stage2 --interactive Bug.hs -dcore-lint
GHCi, version 8.7.20190120: https://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
*** Core Lint errors : in result of Desugar (before optimization) ***
<no location info>: warning:
    In the expression: SomeApply
                         @ k_a1Dz
                         @ (TyCon1 Sing)
                         @ Any
                         (s_a1Bq
                          `cast` (Sub (D:R:ApplyabTyCon1x[0]
                                           <k_a1Dz>_N
                                           <*>_N
                                           <Sing>_N
                                           <a_a1DB>_N ; Sym (D:R:ApplyabTyCon1x[0]
                                                                 <k_a1Dz>_N <*>_N <Sing>_N <Any>_N))
                                  :: Apply (TyCon1 Sing) a_a1DB ~R# Apply (TyCon1 Sing) Any))
    Trans coercion mis-match: D:R:ApplyabTyCon1x[0]
                                  <k_a1Dz>_N <*>_N <Sing>_N <a_a1DB>_N ; Sym (D:R:ApplyabTyCon1x[0]
                                                                                  <k_a1Dz>_N
                                                                                  <*>_N
                                                                                  <Sing>_N
                                                                                  <Any>_N)
      Apply (TyCon1 Sing) a_a1DB
      Sing a_a1DB
      Sing Any
      Apply (TyCon1 Sing) Any
*** Offending Program ***

<elided>

f :: forall k. SomeApply (TyCon1 Sing) -> SomeApply (TyCon1 Sing)
[LclIdX]
f = \ (@ k_a1Dz) (ds_d1EV :: SomeApply (TyCon1 Sing)) ->
      case ds_d1EV of wild_00 { SomeApply @ a_a1DB s_a1Bq ->
      break<0>(s_a1Bq)
      SomeApply
        @ k_a1Dz
        @ (TyCon1 Sing)
        @ Any
        (s_a1Bq
         `cast` (Sub (D:R:ApplyabTyCon1x[0]
                          <k_a1Dz>_N <*>_N <Sing>_N <a_a1DB>_N ; Sym (D:R:ApplyabTyCon1x[0]
                                                                          <k_a1Dz>_N
                                                                          <*>_N
                                                                          <Sing>_N
                                                                          <Any>_N))
                 :: Apply (TyCon1 Sing) a_a1DB ~R# Apply (TyCon1 Sing) Any))
      }

Note that if I change the definition of Sing to this:

data family Sing (a :: k)

Then the error goes away.

Also, if I extend SomeProxy with an additional proxy field:

data SomeApply :: (k ~> Type) -> Type where
  SomeApply :: proxy a -> Apply f a -> SomeApply f

f :: SomeApply (TyCon1 Sing :: k ~> Type)
  -> SomeApply (TyCon1 Sing :: k ~> Type)
f (SomeApply p s)
 = SomeApply p s

Then the error also goes away. Perhaps a being ambiguous plays an important role here?

Possibly related to #16188 (closed) or #16204 (closed).

Trac metadata
Trac field Value
Version 8.7
Type Bug
TypeOfFailure OtherFailure
Priority highest
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

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8.8.1
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bug P::highest Trac import typechecker TypeInType
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Reference: ghc/ghc#16225