#7196: Desugarer needs an extra case for casts in coercions
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.2
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Ganesh (via Darcs) found the code below crashes GHC 7.6rc1, thus:
{{{
ghc-stage1: panic! (the 'impossible' happened)
(GHC version 7.6.0 for x86_64-unknown-linux):
coVarsOfTcCo:Bind
cobox{v agK} [lid]
= cobox{v agY} [lid] `cast` (<main:Patch.WithState{tc reC}
(main:Patch.ModelOf{tc reH} prim{tv
t1h} [tv])
prim{tv t1h} [tv]>
ghc-prim:GHC.Types.~{(w) tc 31Q}
main:Patch.WithState{tc reC}
(main:Patch.ModelOf{tc reH}
(Sym cobox{v agV} [lid]))
<prim{tv t1h} [tv]>)
}}}
Problem is a missing case in `DsBinds.ds_tc_coercion`.
Here's the test case
{{{
{-# LANGUAGE ImpredicativeTypes #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}
module Patch (qc_prim) where
class PrimPatch (prim :: * -> * -> *)
class PrimPatch (PrimOf p) => PrimPatchBase p where
type PrimOf (p :: * -> * -> *) :: * -> * -> *
type TestGenerator thing gen = (forall t ctx . (forall xx yy . thing xx yy
-> t) -> (gen ctx -> t))
type family ModelOf (patch :: * -> * -> *) :: * -> *
data WithState s p x y = WithState {
_wsStartState :: s x
, _wsPatch :: p x y
, _wsEndState :: s y
}
arbitraryThing :: x -> TestGenerator thing (thing x)
arbitraryThing _ f p = f p
qc_prim :: forall prim x y .
(PrimPatch prim
, PrimOf prim ~ prim
) => prim x y -> [()]
qc_prim _ =
concat
[
patch_repo_properties (undefined :: prim x a) "arbitrary"
arbitraryThing'
]
where arbitraryThing' = arbitraryThing (undefined :: a)
patch_repo_properties :: p x y -> String -> TestGenerator (WithState
(ModelOf (PrimOf p)) p) gen -> [()]
patch_repo_properties _ _genname _gen = undefined
}}}
Thanks to Ganesh for isolating it out of Darcs.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7196>
GHC <http://www.haskell.org/ghc/>
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