[GHC] #9582: Associated Type Synonyms do not unfold in InstanceSigs

[GHC]

#9582: Associated Type Synonyms do not unfold in InstanceSigs
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              Reporter:              |            Owner:
  andreas.abel                       |           Status:  new
                  Type:  bug         |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:  InstanceSigs
  (Type checker)                     |  TypeFamilies
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by andreas.abel):

 Relevant snippet: Checking InstanceSigs uses eqType.

 https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcInstDcls.lhs
 {{{#!hs
     -- Check that any type signatures have exactly the right type
     check_inst_sig hs_ty@(L loc _)
        = setSrcSpan loc $
          do { sig_ty <- tcHsSigType (FunSigCtxt sel_name) hs_ty
             ; inst_sigs <- xoptM Opt_InstanceSigs
             ; if inst_sigs then
                 unless (sig_ty `eqType` local_meth_ty)
                        (badInstSigErr sel_name local_meth_ty)
               else
                 addErrTc (misplacedInstSig sel_name hs_ty)
             ; return sig_ty }
 }}}
 Last commit:
 https://github.com/ghc/ghc/commit/7fa2ce2043e2faed2b2b545ba5c1c9958954800a

 According to the documentation, eqType only takes type synonyms into
 account (not type families).

 https://github.com/ghc/ghc/blob/master/compiler/types/Type.lhs

 {{{#!hs
 eqType :: Type -> Type -> Bool
 -- ^ Type equality on source types. Does not look through @newtypes@ or
 -- 'PredType's, but it does look through type synonyms.
 -- Watch out for horrible hack: See Note [Comparison with OpenTypeKind]
 eqType t1 t2 = isEqual $ cmpType t1 t2

 cmpType :: Type -> Type -> Ordering
 -- Watch out for horrible hack: See Note [Comparison with OpenTypeKind]
 cmpType t1 t2 = cmpTypeX rn_env t1 t2
   where
     rn_env = mkRnEnv2 (mkInScopeSet (tyVarsOfType t1 `unionVarSet`
 tyVarsOfType t2))

 cmpTypeX :: RnEnv2 -> Type -> Type -> Ordering  -- Main workhorse
 cmpTypeX env t1 t2 | Just t1' <- coreView t1 = cmpTypeX env t1' t2
                    | Just t2' <- coreView t2 = cmpTypeX env t1 t2'
 -- We expand predicate types, because in Core-land we have
 -- lots of definitions like
 --      fOrdBool :: Ord Bool
 --      fOrdBool = D:Ord .. .. ..
 -- So the RHS has a data type

 cmpTypeX env (TyVarTy tv1)       (TyVarTy tv2)       = rnOccL env tv1
 `compare` rnOccR env tv2
 -- plus congruences...
 }}}
 Where to find the type equality check that honors type family instances?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9582#comment:1>
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