Overlap check for type families

- If two "type instance"s overlap, they right-hand sides must be syntactically
  equal under the overlap substitution.  (Ie, we admit limited overlap, but
  require the system to still be confluent.)

diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index f85f6b9..712ac39 100644 (file)
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -180,19 +180,27 @@ checkForConflicts inst_envs famInst
 
        ; let { matches   = lookupFamInstEnvUnify inst_envs fam tys'
             ; conflicts = [ conflictingFamInst
-                          | match@(conflictingFamInst, _) <- matches
-                          , conflicting fam tys' tycon match 
+                          | match@((conflictingFamInst, _), _) <- matches
+                          , conflicting tycon match 
                           ]
             }
        ; unless (null conflicts) $
           conflictInstErr famInst (head conflicts)
        }
   where
-    -- In the case of data/newtype instances, any overlap is a conflict (as
-    -- these instances imply injective type mappings).
-    conflicting _   _    tycon _                 | isAlgTyCon tycon = True
-    conflicting fam tys' tycon (subst, cFamInst) | otherwise     =
-      panic "FamInst.checkForConflicts: overlap check for indexed synonyms is still missing"
+      -- * In the case of data family instances, any overlap is fundamentally a 
+      --   conflict (as these instances imply injective type mappings).
+      -- * In the case of type family instances, overlap is admitted as long as 
+      --   the right-hand sides of the overlapping rules coincide under the
+      --   overlap substitution.  We require that they are syntactically equal;
+      --   anything else would be difficult to test for at this stage.
+    conflicting tycon1 ((famInst2, _), subst) 
+      | isAlgTyCon tycon1 = True
+      | otherwise         = not (rhs1 `tcEqType` rhs2)
+      where
+        tycon2 = famInstTyCon famInst2
+        rhs1   = substTy subst $ synTyConType tycon1
+        rhs2   = substTy subst $ synTyConType tycon2
 
 conflictInstErr famInst conflictingFamInst
   = addFamInstLoc famInst $

diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index ee55583..7c06555 100644 (file)
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -277,7 +277,7 @@ indexed synonyms and we don't want to slow that down by needless unification.
 
 \begin{code}
 lookupFamInstEnvUnify :: (FamInstEnv, FamInstEnv) -> TyCon -> [Type]
-                     -> [(FamInstMatch)]
+                     -> [(FamInstMatch, TvSubst)]
 lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
   | not (isOpenTyCon fam) 
   = []
@@ -318,7 +318,7 @@ lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
         case tcUnifyTys bind_fn tpl_tys tys of
            Just subst -> let rep_tys = substTyVars subst (tyConTyVars tycon)
                           in
-                          (item, rep_tys) : find rest
+                          ((item, rep_tys), subst) : find rest
            Nothing    -> find rest
 
 -- See explanation at @InstEnv.bind_fn@.