Separate transCloVarSet from fixVarSet

I wasn't clear about the distinction before, and that led to a bug
when I refactored FunDeps.oclose to use transCloVarSet; it should
use fixVarSet.

compiler/basicTypes/VarSet.hs

@@ -16,8 +16,8 @@ module VarSet (
         unionVarSet, unionVarSets, mapUnionVarSet,
         intersectVarSet, intersectsVarSet, disjointVarSet,
         isEmptyVarSet, delVarSet, delVarSetList, delVarSetByKey,
-        minusVarSet, foldVarSet, filterVarSet, 
-        transCloVarSet,
+        minusVarSet, foldVarSet, filterVarSet,
+        transCloVarSet, fixVarSet,
         lookupVarSet, mapVarSet, sizeVarSet, seqVarSet,
         elemVarSetByKey, partitionVarSet
     ) where
@@ -110,13 +110,28 @@ intersectsVarSet s1 s2 = not (s1 `disjointVarSet` s2)
 disjointVarSet   s1 s2 = isEmptyVarSet (s1 `intersectVarSet` s2)
 subVarSet        s1 s2 = isEmptyVarSet (s1 `minusVarSet` s2)
 
+fixVarSet :: (VarSet -> VarSet)   -- Map the current set to a new set
+          -> VarSet -> VarSet
+-- (fixVarSet f s) repeatedly applies f to the set s, 
+-- until it reaches a fixed point.
+fixVarSet fn vars
+  | new_vars `subVarSet` vars = vars
+  | otherwise                 = fixVarSet fn new_vars
+  where
+    new_vars = fn vars
+
 transCloVarSet :: (VarSet -> VarSet)
                   -- Map some variables in the set to
                   -- extra variables that should be in it
                -> VarSet -> VarSet
--- (transCloVarSet f s) repeatedly applies f to the set s, adding any
--- new variables to s that it finds thereby, until it reaches a fixed
--- point.  The actual algorithm is a bit more efficient.
+-- (transCloVarSet f s) repeatedly applies f to new candidates, adding any
+-- new variables to s that it finds thereby, until it reaches a fixed point.
+--
+-- The function fn could be (Var -> VarSet), but we use (VarSet -> VarSet)
+-- for efficiency, so that the test can be batched up.
+-- It's essential that fn will work fine if given new candidates
+-- one at at time; ie  fn {v1,v2} = fn v1 `union` fn v2
+-- Use fixVarSet if the function needs to see the whole set all at once
 transCloVarSet fn seeds
   = go seeds seeds
   where
@@ -124,7 +139,7 @@ transCloVarSet fn seeds
        -> VarSet  -- Work-list; un-processed subset of accumulating result
        -> VarSet
     -- Specification: go acc vs = acc `union` transClo fn vs
-   
+
     go acc candidates
        | isEmptyVarSet new_vs = acc
        | otherwise            = go (acc `unionVarSet` new_vs) new_vs

compiler/typecheck/FunDeps.hs

@@ -446,7 +446,7 @@ oclose :: [PredType] -> TyVarSet -> TyVarSet
 -- See Note [The liberal coverage condition]
 oclose preds fixed_tvs
   | null tv_fds = fixed_tvs -- Fast escape hatch for common case.
-  | otherwise   = transCloVarSet extend fixed_tvs
+  | otherwise   = fixVarSet extend fixed_tvs
   where
     extend fixed_tvs = foldl add fixed_tvs tv_fds
        where