From d4b980043211a2b1e7a24a6efd9e21e9853dcde0 Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Thu, 11 Mar 2004 14:29:59 +0000
Subject: [PATCH] [project @ 2004-03-11 14:29:59 by simonpj] Add sanity check

---
 ghc/compiler/types/InstEnv.lhs |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/ghc/compiler/types/InstEnv.lhs b/ghc/compiler/types/InstEnv.lhs
index cc75492..905cde2 100644
--- a/ghc/compiler/types/InstEnv.lhs
+++ b/ghc/compiler/types/InstEnv.lhs
@@ -299,7 +299,10 @@ lookup_inst_env env key_cls key_tys
 	  Nothing 
 		-- Does not match, so next check whether the things unify
 		-- [see notes about overlapping instances above]
-	   -> case unifyTyListsX (key_vars `unionVarSet` tpl_tyvars) key_tys tpl of
+	   -> ASSERT( not (key_vars `intersectsVarSet` tpl_tyvars) )
+		-- Unification will break badly if the variables overlap
+		-- They shouldn't because we allocate separate uniques for them
+	      case unifyTyListsX (key_vars `unionVarSet` tpl_tyvars) key_tys tpl of
 	        Just _   -> find rest ms (dfun_id:us)
 	        Nothing  -> find rest ms us
 
-- 
1.7.10.4