[project @ 2005-01-05 17:49:38 by simonpj]

Add assert

diff --git a/ghc/compiler/types/InstEnv.lhs b/ghc/compiler/types/InstEnv.lhs
index 5b41bf9..46ab2cb 100644 (file)
--- 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