[project @ 2004-03-11 14:29:59 by simonpj]

Add sanity check

diff --git a/ghc/compiler/types/InstEnv.lhs b/ghc/compiler/types/InstEnv.lhs
index cc75492..905cde2 100644 (file)
--- 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