X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcGadt.lhs;h=e45d6bd53a5c887999e5bdf1ec74220711e4e0ad;hp=b556e89e3273ed15e45c2ad32e4d8479f7979933;hb=30c122df62ec75f9ed7f392f24c2925675bf1d06;hpb=f8c52d7fde2d7408b4f734251c373f8d3e2c558e

diff --git a/compiler/typecheck/TcGadt.lhs b/compiler/typecheck/TcGadt.lhs
index b556e89..e45d6bd 100644
--- a/compiler/typecheck/TcGadt.lhs
+++ b/compiler/typecheck/TcGadt.lhs
@@ -41,6 +41,7 @@ import Outputable
 import TcType
 import Unique
 import UniqFM
+import FastString
 \end{code}
 
 
@@ -237,11 +238,15 @@ fixTvCoEnv in_scope env
       -- then use transitivity with the original coercion
 
 -----------------------------
+-- XXX Can we do this more nicely, by exploiting laziness?
+-- Or avoid needing it in the first place?
 fixTvSubstEnv :: InScopeSet -> TvSubstEnv -> TvSubstEnv
-fixTvSubstEnv in_scope env
-  = fixpt 
+fixTvSubstEnv in_scope env = f env
   where
-    fixpt = mapVarEnv (substTy (mkTvSubst in_scope fixpt)) env
+    f e = let e' = mapUFM (substTy (mkTvSubst in_scope e)) e
+          in if and $ eltsUFM $ intersectUFM_C tcEqType e e'
+             then e
+             else f e'
 
 ----------------------------
 tryToBind :: TyVarSet -> TyVar -> BindFlag