id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
7196	Desugarer needs an extra case for casts in coercions	simonpj		"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."	bug	closed	normal	7.6.1	Compiler	7.4.2	fixed		ganesh@…	Unknown/Multiple	Unknown/Multiple	None/Unknown	Unknown	typecheck/should_compile/T7196