Tidy tyvar OccNames in TcTyClDecl

We want the universal and existential tyvars of a data constructor to
have distinct OccNames.  It's confusing if they don't (in error messages,
for example), and with the current way of generating IfaceSyn, it actally
generates bogus interface files.  (Which bit Roman.)

When IfaceSyn is full of Names, this won't matter so much, but it still
seems cleaner.

This patch adds a 'tidy' step to the generation of DataCon type 
variables in TcTyClDecls.tcResultType 

compiler/basicTypes/DataCon.lhs

@@ -260,6 +260,9 @@ data DataCon
 		-- [This is a change (Oct05): previously, vanilla datacons guaranteed to
 		--  have the same type variables as their parent TyCon, but that seems ugly.]
 
+	-- INVARIANT: the UnivTyVars and ExTyVars all have distinct OccNames
+	-- Reason: less confusing, and easier to generate IfaceSyn
+
 	dcEqSpec :: [(TyVar,Type)],	-- Equalities derived from the result type, 
 					-- *as written by the programmer*
 		-- This field allows us to move conveniently between the two ways

compiler/typecheck/TcTyClsDecls.lhs

@@ -12,10 +12,9 @@ module TcTyClsDecls (
 
 import HsSyn		( TyClDecl(..),  HsConDetails(..), HsTyVarBndr(..),
 			  ConDecl(..),   Sig(..), NewOrData(..), ResType(..),
-			  tyClDeclTyVars, isSynDecl, isClassDecl, isIdxTyDecl,
+			  tyClDeclTyVars, isSynDecl, isIdxTyDecl,
 			  isKindSigDecl, hsConArgs, LTyClDecl, tcdName,
-			  hsTyVarName, LHsTyVarBndr, LHsType, HsType(..),
-			  mkHsAppTy
+			  hsTyVarName, LHsTyVarBndr, LHsType
 			)
 import HsTypes          ( HsBang(..), getBangStrictness, hsLTyVarNames )
 import BasicTypes	( RecFlag(..), StrictnessMark(..) )
@@ -38,9 +37,9 @@ import TcMType		( newKindVar, checkValidTheta, checkValidType,
 			  -- checkFreeness, 
 			  UserTypeCtxt(..), SourceTyCtxt(..) ) 
 import TcType		( TcKind, TcType, Type, tyVarsOfType, mkPhiTy,
-			  mkArrowKind, liftedTypeKind, mkTyVarTys, 
-			  tcSplitSigmaTy, tcEqTypes, tcGetTyVar_maybe )
-import Type		( PredType(..), splitTyConApp_maybe, mkTyVarTy,
+			  mkArrowKind, liftedTypeKind, 
+			  tcSplitSigmaTy, tcGetTyVar_maybe )
+import Type		( splitTyConApp_maybe, 
                           newTyConInstRhs, isLiftedTypeKind, Kind,
                           splitKindFunTys, mkArrowKinds
 			  -- pprParendType, pprThetaArrow
@@ -51,22 +50,23 @@ import TyCon		( TyCon, AlgTyConRhs( AbstractTyCon, OpenDataTyCon,
 					      OpenNewTyCon ), 
 			  SynTyConRhs( OpenSynTyCon, SynonymTyCon ),
 			  tyConDataCons, mkForeignTyCon, isProductTyCon,
-			  isRecursiveTyCon, isOpenTyCon,
+			  isRecursiveTyCon, 
 			  tyConStupidTheta, synTyConRhs, isSynTyCon, tyConName,
                           isNewTyCon, isDataTyCon, tyConKind, 
 			  setTyConArgPoss )
 import DataCon		( DataCon, dataConUserType, dataConName, 
 			  dataConFieldLabels, dataConTyCon, dataConAllTyVars,
 			  dataConFieldType, dataConResTys )
-import Var		( TyVar, idType, idName )
+import Var		( TyVar, idType, idName, tyVarName, setTyVarName )
 import VarSet		( elemVarSet, mkVarSet )
-import Name		( Name, getSrcLoc )
+import Name		( Name, getSrcLoc, tidyNameOcc, getOccName )
+import OccName		( initTidyOccEnv, tidyOccName )
 import Outputable
 import Maybe		( isJust, fromJust, isNothing, catMaybes )
 import Maybes		( expectJust )
 import Monad		( unless )
 import Unify		( tcMatchTys, tcMatchTyX )
-import Util		( zipLazy, isSingleton, notNull, sortLe )
+import Util		( zipLazy, isSingleton, notNull, sortLe, mapAccumL )
 import List		( partition, elemIndex )
 import SrcLoc		( Located(..), unLoc, getLoc, srcLocSpan, 
 			  srcSpanStart )
@@ -797,6 +797,7 @@ tcConDecl unbox_strict DataType tycon tc_tvs	-- Data types
     { ctxt' <- tcHsKindedContext ctxt
     ; (univ_tvs, ex_tvs, eq_preds, data_tc) <- tcResultType tycon tc_tvs tvs' res_ty
     ; let 
+	-- Tiresome: tidy the tyvar binders, since tc_tvs and tvs' may have the same OccNames
 	tc_datacon is_infix field_lbls btys
 	  = do { let bangs = map getBangStrictness btys
 	       ; arg_tys <- mappM tcHsBangType btys
@@ -823,7 +824,7 @@ tcResultType :: TyCon
 	     -> [TyVar] 	-- where MkT :: forall a b c. ...
 	     -> ResType Name
 	     -> TcM ([TyVar],	 	-- Universal
-		     [TyVar],		-- Existential
+		     [TyVar],		-- Existential (distinct OccNames from univs)
 		     [(TyVar,Type)],	-- Equality predicates
 		     TyCon)		-- TyCon given in the ResTy
 	-- We don't check that the TyCon given in the ResTy is
@@ -843,8 +844,8 @@ tcResultType _ tc_tvs dc_tvs (ResTyGADT res_ty)
 	--	([a,z,c], [x,y], [a:=:(x,y), c:=:z], T)
 
   = do	{ (dc_tycon, res_tys) <- tcLHsConResTy res_ty
-		-- NB: tc_tvs and dc_tvs are distinct
-	; let univ_tvs = choose_univs [] tc_tvs res_tys
+
+	; let univ_tvs = choose_univs [] tidy_tc_tvs res_tys
 		-- Each univ_tv is either a dc_tv or a tc_tv
 	      ex_tvs = dc_tvs `minusList` univ_tvs
 	      eq_spec = [ (tv, ty) | (tv,ty) <- univ_tvs `zip` res_tys, 
@@ -861,7 +862,19 @@ tcResultType _ tc_tvs dc_tvs (ResTyGADT res_ty)
 	| otherwise
 	= tc_tv : choose_univs used tc_tvs res_tys
 
--------------------
+	-- NB: tc_tvs and dc_tvs are distinct, but
+	-- we want them to be *visibly* distinct, both for
+	-- interface files and general confusion.  So rename
+	-- the tc_tvs, since they are not used yet (no 
+	-- consequential renaming needed)
+    init_occ_env     = initTidyOccEnv (map getOccName dc_tvs)
+    (_, tidy_tc_tvs) = mapAccumL tidy_one init_occ_env tc_tvs
+    tidy_one env tv  = (env', setTyVarName tv (tidyNameOcc name occ'))
+	      where
+		 name = tyVarName tv
+		 (env', occ') = tidyOccName env (getOccName name) 
+
+	      -------------------
 argStrictness :: Bool		-- True <=> -funbox-strict_fields
 	      -> TyCon -> [HsBang]
 	      -> [TcType] -> [StrictnessMark]