[GHC] #7805: Panic: promoteType with higher-rank datatype

[GHC]

#7805: Panic: promoteType with higher-rank datatype
-------------------------------+--------------------------------------------
Reporter:  goldfire            |          Owner:                  
    Type:  bug                 |         Status:  new             
Priority:  normal              |      Component:  Compiler        
 Version:  7.7                 |       Keywords:  DataKinds       
      Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple
 Failure:  Compile-time crash  |      Blockedby:                  
Blocking:                      |        Related:                  
-------------------------------+--------------------------------------------

Comment(by goldfire):

 The following change fixes the problem and passes the {{{polykinds}}}
 tests:

 {{{
 diff --git a/compiler/typecheck/TcTyDecls.lhs
 b/compiler/typecheck/TcTyDecls.lhs
 index 99ee065..690bea6 100644
 --- a/compiler/typecheck/TcTyDecls.lhs
 +++ b/compiler/typecheck/TcTyDecls.lhs
 @@ -506,21 +506,21 @@ isPromotableType :: NameSet -> Type -> Bool
  -- Must line up with DataCon.promoteType
  -- But the function lives here because we must treat the
  -- *recursive* tycons as promotable
  isPromotableType rec_tcs ty
    = case splitForAllTys ty of
 -      (_, rho) -> go rho
 +      (tvs, rho) -> null tvs && go rho
    where
      go (TyConApp tc tys)
        | tys `lengthIs` tyConArity tc
        ,  tyConName tc `elemNameSet` rec_tcs
        || isJust (promotableTyCon_maybe tc)
                         = all go tys
        | otherwise      = False
      go (FunTy arg res) = go arg && go res
      go (AppTy arg res) = go arg && go res
 -    go (ForAllTy _ ty) = go ty
 +    go (ForAllTy {})   = False
      go (TyVarTy {})    = True
      go (LitTy {})      = False
  \end{code}
  }}}

 The change looks right to me. The change to {{{ForAllTy}}} is unnecessary
 to fix the original bug, but it is easy to create another test case that
 requires the {{{ForAllTy}}} fix. Is there a reason that the quantified
 variables were ignored previously?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7805#comment:1>
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