[GHC] #7861: deferred type error with rankNTypes

[GHC]

#7861: deferred type error with rankNTypes
-------------------------------+--------------------------------------------
Reporter:  guest               |          Owner:                
    Type:  bug                 |         Status:  new           
Priority:  normal              |      Component:  Compiler      
 Version:  7.6.2               |       Keywords:                
      Os:  Linux               |   Architecture:  x86_64 (amd64)
 Failure:  Compile-time crash  |      Blockedby:                
Blocking:                      |        Related:                
-------------------------------+--------------------------------------------
 {-# LANGUAGE RankNTypes #-}

 type A a = forall b. (a -> b) -> b

 doA :: (a -> b) -> A a -> b
 doA f l = l f

 -- f :: A a -> [a] -> [a]   -- correct type
 f :: A a -> a               -- wrong type
 f = doA (:)

 main = return ()

 -- compiled with wrong type and -fdefer-type-errors
 -- gives ghc panic
 -- F.hs:10:5: Warning:
 --     Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0]
 --     Expected type: A a -> a
 --       Actual type: A a0 -> a
 --     In the return type of a call of `doA'
 --     In the expression: doA (:)
 --     In an equation for `f': f = doA (:)
 --
 -- F.hs:10:9: Warning:
 --     Couldn't match type `a' with `[a0] -> [a0]'
 --       `a' is a rigid type variable bound by
 --           the type signature for f :: A a -> a at F.hs:9:6
 --     Expected type: a0 -> a
 --       Actual type: a0 -> [a0] -> [a0]
 --     In the first argument of `doA', namely `(:)'
 --     In the expression: doA (:)
 --     In an equation for `f': f = doA (:)
 -- ghc: panic! (the 'impossible' happened)
 --   (GHC version 7.6.2 for x86_64-unknown-linux):
 --      evTermCoercion
 --     error @ghc-prim:GHC.Prim.Any{(w) tc 31N}
 --              ghc-prim:GHC.Prim.*{(w) tc 34d}
 --            ghc-prim:GHC.Types.~{(w) tc 31Q} ([ghc-prim:GHC.Prim.Any{(w)
 tc 31N}
 --                                                 ghc-prim:GHC.Prim.*{(w)
 tc 34d}]
 --                                              -> [ghc-
 prim:GHC.Prim.Any{(w) tc 31N}
 --                                                    ghc-
 prim:GHC.Prim.*{(w) tc 34d}])
 --           F.hs:10:5:
 --     Occurs check: cannot construct the infinite type: a0 = [a0] -> [a0]
 --     Expected type: A a -> a
 --       Actual type: A a0 -> a
 --     In the return type of a call of `doA'
 --     In the expression: doA (:)
 --     In an equation for `f': f = doA (:)
 -- (deferred type error)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7861>
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