Commit b70fc87d authored by's avatar

Fix Trac #3423: missed instantiation for newtype-derived instances

Somehow I'd forgotten to instantiate the coercion that is stored in a
'NewtypeDerived' constructor in an InstInfo.  The necessary code is
in TcInstDcls.tc_inst_decl2.

The result was ghc: panic! (the 'impossible' happened)
   (GHC version 6.10.3 for x86_64-unknown-linux):
   No match in record selector Var.tcTyVarDetails
because we were looking at an (uninstantiated) TyVar instead of
an (instantiated) TcTyVar.
parent b0ec2cb2
......@@ -1387,6 +1387,7 @@ genInst standalone_deriv oflag spec
-- When dealing with the deriving clause
-- co1 : N [(b,b)] ~ R1:N (b,b)
-- co2 : R1:N (b,b) ~ Tree (b,b)
-- co : N [(b,b)] ~ Tree (b,b)
genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
genDerivBinds loc fix_env clas tycon
......@@ -600,9 +600,10 @@ tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
-- see Note [Newtype deriving superclasses] in TcDeriv.lhs
tc_inst_decl2 dfun_id (NewTypeDerived coi)
= do { let rigid_info = InstSkol
origin = SigOrigin rigid_info
inst_ty = idType dfun_id
= do { let rigid_info = InstSkol
origin = SigOrigin rigid_info
inst_ty = idType dfun_id
inst_tvs = fst (tcSplitForAllTys inst_ty)
; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
-- inst_head_ty is a PredType
......@@ -615,7 +616,13 @@ tc_inst_decl2 dfun_id (NewTypeDerived coi)
(rep_ty, wrapper)
= case coi of
IdCo -> (last_ty, idHsWrapper)
ACo co -> (snd (coercionKind co), WpCast (mk_full_coercion co))
ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
-- NB: the free variable of coi are bound by the
-- universally quantified variables of the dfun_id
-- This is weird, and maybe we should make NewTypeDerived
-- carry a type-variable list too; but it works fine
-- mk_full_coercion
......@@ -736,7 +743,7 @@ tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
this_dict dfun_id
prag_fn monobinds
; (meth_exprs, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
mapAndUnzipM tc_meth op_items
mapAndUnzipM tc_meth op_items
-- Figure out bindings for the superclass context
-- Don't include this_dict in the 'givens', else
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