Fix Trac #3219: type of a record update

Record updates are amazingly hard to typecheck right.  This is one place
where GHC's policy of typechecking the original source is much harder than
desugaring and typechecking that!

Anyway, the bug here is that to compute the 'fixed' type variables I was
only looking at one constructor rather than all the relevant_cons

Test is typecheck/should_compile/T3219, which GHC 6.10 barfs on (with Lint).

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index b255fdb..b101cb5 100644 (file)
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -408,16 +408,20 @@ Note [Type of a record update]
 The main complication with RecordUpd is that we need to explicitly
 handle the *non-updated* fields.  Consider:
 
-       data T a b = MkT1 { fa :: a, fb :: b }
-                  | MkT2 { fa :: a, fc :: Int -> Int }
-                  | MkT3 { fd :: a }
+       data T a b c = MkT1 { fa :: a, fb :: (b,c) }
+                    | MkT2 { fa :: a, fb :: (b,c), fc :: c -> c }
+                    | MkT3 { fd :: a }
        
-       upd :: T a b -> c -> T a c
+       upd :: T a b c -> (b',c) -> T a b' c
        upd t x = t { fb = x}
 
-The type signature on upd is correct (i.e. the result should not be (T a b))
-because upd should be equivalent to:
+The result type should be (T a b' c)
+not (T a b c),   because 'b' *is not* mentioned in a non-updated field
+not (T a b' c'), becuase 'c' *is*     mentioned in a non-updated field
+NB that it's not good enough to look at just one constructor; we must
+look at them all; cf Trac #3219
 
+After all, upd should be equivalent to:
        upd t x = case t of 
                        MkT1 p q -> MkT1 p x
                        MkT2 a b -> MkT2 p b
@@ -425,9 +429,11 @@ because upd should be equivalent to:
 
 So we need to give a completely fresh type to the result record,
 and then constrain it by the fields that are *not* updated ("p" above).
+We call these the "fixed" type variables, and compute them in getFixedTyVars.
 
 Note that because MkT3 doesn't contain all the fields being updated,
-its RHS is simply an error, so it doesn't impose any type constraints
+its RHS is simply an error, so it doesn't impose any type constraints.
+Hence the use of 'relevant_cont'.
 
 Note [Implict type sharing]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -490,11 +496,10 @@ In the outgoing (HsRecordUpd scrut binds cons in_inst_tys out_inst_tys):
        
 \begin{code}
 tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
-  = do {
+  = ASSERT( notNull upd_fld_names )
+    do {
        -- STEP 0
        -- Check that the field names are really field names
-         let upd_fld_names = hsRecFields rbinds
-       ; MASSERT( notNull upd_fld_names )
        ; sel_ids <- mapM tcLookupField upd_fld_names
                        -- The renamer has already checked that
                        -- selectors are all in scope
@@ -532,11 +537,11 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
        -- STEP 3    Note [Criteria for update]
        -- Check that each updated field is polymorphic; that is, its type
        -- mentions only the universally-quantified variables of the data con
-       ; let flds_w_tys = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
-             (upd_flds_w_tys, fixed_flds_w_tys) = partition is_updated flds_w_tys
+       ; let flds1_w_tys = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
+             (upd_flds1_w_tys, fixed_flds1_w_tys) = partition is_updated flds1_w_tys
              is_updated (fld,ty) = fld `elem` upd_fld_names
 
-             bad_upd_flds = filter bad_fld upd_flds_w_tys
+             bad_upd_flds = filter bad_fld upd_flds1_w_tys
              con1_tv_set = mkVarSet con1_tvs
              bad_fld (fld, ty) = fld `elem` upd_fld_names &&
                                      not (tyVarsOfType ty `subVarSet` con1_tv_set)
@@ -546,14 +551,14 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
        -- Figure out types for the scrutinee and result
        -- Both are of form (T a b c), with fresh type variables, but with
        -- common variables where the scrutinee and result must have the same type
-       -- These are variables that appear anywhere *except* in the updated fields
-       ; let common_tvs = exactTyVarsOfTypes (map snd fixed_flds_w_tys)
-                          `unionVarSet` constrainedTyVars con1_tvs relevant_cons
-             is_common_tv tv = tv `elemVarSet` common_tvs
-
+       -- These are variables that appear in *any* arg of *any* of the relevant constructors
+        -- *except* in the updated fields
+       -- 
+       ; let fixed_tvs = getFixedTyVars con1_tvs relevant_cons
+             is_fixed_tv tv = tv `elemVarSet` fixed_tvs
              mk_inst_ty tv result_inst_ty 
-               | is_common_tv tv = return result_inst_ty           -- Same as result type
-               | otherwise       = newFlexiTyVarTy (tyVarKind tv)  -- Fresh type, of correct kind
+               | is_fixed_tv tv = return result_inst_ty            -- Same as result type
+               | otherwise      = newFlexiTyVarTy (tyVarKind tv)  -- Fresh type, of correct kind
 
        ; (_, result_inst_tys, result_inst_env) <- tcInstTyVars con1_tvs
        ; scrut_inst_tys <- zipWithM mk_inst_ty con1_tvs result_inst_tys
@@ -585,17 +590,25 @@ tcExpr expr@(RecordUpd record_expr rbinds _ _ _) res_ty
        ; return (mkHsWrap co_fn (RecordUpd (mkLHsWrap scrut_co record_expr') rbinds'
                                        relevant_cons scrut_inst_tys result_inst_tys)) }
   where
-    constrainedTyVars :: [TyVar] -> [DataCon] -> TyVarSet
-    -- Universally-quantified tyvars that appear in any of the 
-    -- *implicit* arguments to the constructor
+    upd_fld_names = hsRecFields rbinds
+
+    getFixedTyVars :: [TyVar] -> [DataCon] -> TyVarSet
     -- These tyvars must not change across the updates
-    -- See Note [Implict type sharing]
-    constrainedTyVars tvs1 cons
+    getFixedTyVars tvs1 cons
       = mkVarSet [tv1 | con <- cons
-                     , let (tvs, theta, _, _) = dataConSig con
-                           bad_tvs = tyVarsOfTheta theta
+                     , let (tvs, theta, arg_tys, _) = dataConSig con
+                           flds = dataConFieldLabels con
+                           fixed_tvs = exactTyVarsOfTypes fixed_tys
+                                   -- fixed_tys: See Note [Type of a record update]
+                                       `unionVarSet` tyVarsOfTheta theta 
+                                   -- Universally-quantified tyvars that appear in any of the 
+                                   -- *implicit* arguments to the constructor are fixed
+                                   -- See Note [Implict type sharing]
+                                       
+                           fixed_tys = [ty | (fld,ty) <- zip flds arg_tys
+                                            , not (fld `elem` upd_fld_names)]
                       , (tv1,tv) <- tvs1 `zip` tvs     -- Discards existentials in tvs
-                     , tv `elemVarSet` bad_tvs ]
+                     , tv `elemVarSet` fixed_tvs ]
 \end{code}
 
 %************************************************************************