From 492303f32eb179c14a095aa175541bdbff486179 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Tue, 14 Dec 2010 18:03:44 +0000
Subject: [PATCH] Fix Trac #3731: more superclass subtlety (sigh)

I will add more comments, but I want to commit this tonight,
so the overnight builds get it.
---
 compiler/typecheck/TcCanonical.lhs |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/compiler/typecheck/TcCanonical.lhs b/compiler/typecheck/TcCanonical.lhs
index 7fdb63e..60d1836 100644
--- a/compiler/typecheck/TcCanonical.lhs
+++ b/compiler/typecheck/TcCanonical.lhs
@@ -320,7 +320,10 @@ happen.
 newSCWorkFromFlavored :: EvVar -> CtFlavor -> Class -> [Xi] -> TcS CanonicalCts
 -- Returns superclasses, see Note [Adding superclasses]
 newSCWorkFromFlavored ev orig_flavor cls xis 
-  = do { let (tyvars, sc_theta, _, _) = classBigSig cls 
+  | isEmptyVarSet (tyVarsOfTypes xis)
+  = return emptyCCan
+  | otherwise
+  = do { let (tyvars, sc_theta, _, _) = classBigSig cls
              sc_theta1 = substTheta (zipTopTvSubst tyvars xis) sc_theta
        ; sc_vars <- zipWithM inst_one sc_theta1 [0..]
        ; mkCanonicals flavor sc_vars }
-- 
1.7.10.4