Fix a small bug when comparing patterns for equality

diff --git a/compiler/specialise/SpecConstr.lhs b/compiler/specialise/SpecConstr.lhs
index 99d51f1..1cdeec1 100644 (file)
--- a/compiler/specialise/SpecConstr.lhs
+++ b/compiler/specialise/SpecConstr.lhs
@@ -783,8 +783,13 @@ specialise env calls (fn, rhs, arg_occs)
        ; let good_pats :: [([Var], [CoreArg])]
              good_pats = catMaybes mb_pats
              in_scope = mkInScopeSet $ unionVarSets $
-                        [ exprsFreeVars pats `delVarSetList` vs 
+                        [ exprsFreeVars pats
                         | (vs,pats) <- good_pats ]
+               -- This in-scope set is used when matching to see if
+               -- we have identical patterns.  We want to treat the
+               -- forall'd variables of each pattern as "in scope",
+               -- because each in turn serves as the match target for
+               -- a matchN call.  So don't remove the 'vs' from the free vars!
              uniq_pats = nubBy (same_pat in_scope) good_pats
 --     ; pprTrace "specialise" (vcat [ppr fn <+> ppr arg_occs,
 --                                     text "calls" <+> ppr all_calls,
@@ -802,8 +807,8 @@ specialise env calls (fn, rhs, arg_occs)
   where
        -- Two pats are the same if they match both ways
     same_pat in_scope (vs1,as1)(vs2,as2)
-        =  isJust (matchN in_scope vs1 as1 as2)
-        && isJust (matchN in_scope vs2 as2 as1)
+       =  isJust (matchN in_scope vs1 as1 as2)
+       && isJust (matchN in_scope vs2 as2 as1)
 
 callToPats :: InScopeEnv -> [ArgOcc] -> Call
           -> UniqSM (Maybe ([Var], [CoreExpr]))