From c82fa23e85ccb3fd6224fd446811a773cf6028c1 Mon Sep 17 00:00:00 2001
From: simonpj <unknown>
Date: Wed, 5 Jan 2005 17:49:38 +0000
Subject: [PATCH] [project @ 2005-01-05 17:49:38 by simonpj] Add assert

---
 ghc/compiler/types/InstEnv.lhs |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/ghc/compiler/types/InstEnv.lhs b/ghc/compiler/types/InstEnv.lhs
index 5b41bf9..46ab2cb 100644
--- a/ghc/compiler/types/InstEnv.lhs
+++ b/ghc/compiler/types/InstEnv.lhs
@@ -322,7 +322,13 @@ lookup_inst_env env key_cls key_tys key_all_tvs
 	  Nothing 
 		-- Does not match, so next check whether the things unify
 		-- [see notes about overlapping instances above]
-	   -> case tcUnifyTys bind_fn tpl key_tys of
+	   -> ASSERT2( not (tyVarsOfTypes key_tys `intersectsVarSet` tpl_tyvars),
+		       (ppr key_cls <+> ppr key_tys <+> ppr key_all_tvs) $$
+		       (ppr dfun_id <+> ppr tpl_tyvars <+> ppr tpl)
+		      )
+		-- Unification will break badly if the variables overlap
+		-- They shouldn't because we allocate separate uniques for them
+	      case tcUnifyTys bind_fn tpl key_tys of
 	        Just _   -> find rest ms (dfun_id:us)
 	        Nothing  -> find rest ms us
 
-- 
1.7.10.4