[GHC] #11371: Bogus in-scope set in substitutions

[GHC]

#11371: Bogus in-scope set in substitutions
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        Reporter:  simonpj           |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11360            |  Differential Rev(s):  phab:D1792,
       Wiki Page:                    |  phab:D1801, phab:D1802
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Comment (by niteria):

 I have a version that performs as well as the old code on T5030:

 {{{
 piResultTys :: Type -> [Type] -> Type
 piResultTys ty args = go_fast ty args
   where
     go_fast ty [] = ty
     go_fast (ForAllTy (Anon _) res) (_:args') = go_fast res args'
     go_fast ty args =
       go (mkEmptyTCvSubst $ mkInScopeSet $ tyCoVarsOfTypes (ty:args)) ty
 args
       -- The free vars of 'ty' and 'args' need to be in scope to satisfy
 the
       -- invariant in Note [The substitution invariant] in TyCoRep.

     go subst ty [] = substTy subst ty
     go subst ty args | Just ty' <- coreView ty = go subst ty' args
     go subst (ForAllTy bndr res) (arg:args')
       = case bndr of
           Anon _     -> go subst                         res args'
           Named tv _ -> go (extendTCvSubst subst tv arg) res args'
     go subst (TyVarTy tv) args
         -- Deals with piResultTys (forall a. a) [forall b.b, Int]
       = go subst (substTyVar subst tv) args
         -- The previous version used empty_subst here,
         -- I'm not sure if it matters for correctness
     go _subst _ty _args = panic "piResultTys"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371#comment:44>
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