Two improvements to boxy matching

I can't quite remember what provoked these two changes, but they are in my
tree.
	One improves boxy_match (which failed unnecessarily)
	One fixes boxy_lub (which was assymetrical)

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index d7399f5..709b7c4 100644 (file)
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -616,6 +616,12 @@ boxy_match tmpl_tvs orig_tmpl_ty boxy_tvs orig_boxy_ty subst
                        Nothing -> orig_boxy_ty
                        Just ty -> ty `boxyLub` orig_boxy_ty
 
+    go _ (TyVarTy tv) | isMetaTyVar tv
+       = subst         -- Don't fail if the template has more info than the target!
+                       -- Otherwise, with tmpl_tvs = [a], matching (a -> Int) ~ (Bool -> beta)
+                       -- would fail to instantiate 'a', because the meta-type-variable
+                       -- beta is as yet un-filled-in
+       
     go _ _ = emptyTvSubst      -- It's important to *fail* by returning the empty substitution
        -- Example:  Tree a ~ Maybe Int
        -- We do not want to bind (a |-> Int) in pre-matching, because that can give very
@@ -644,6 +650,10 @@ boxyLub orig_ty1 orig_ty2
       | isTcTyVar tv1, isBoxyTyVar tv1         -- choose ty2 if ty2 is a box
       = orig_ty2       
 
+    go ty1 (TyVarTy tv2)               -- Symmetrical case
+      | isTcTyVar tv2, isBoxyTyVar tv2
+      = orig_ty1       
+
        -- Look inside type synonyms, but only if the naive version fails
     go ty1 ty2 | Just ty1' <- tcView ty1 = go ty1' ty2
               | Just ty2' <- tcView ty1 = go ty1 ty2'