X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcCanonical.lhs;h=60d1836ee1a82bcc3ee9d6b011b464b44432ca26;hb=492303f32eb179c14a095aa175541bdbff486179;hp=7fdb63ed855ea8e19260c204282fcd42319baa23;hpb=62f76a3cbced691b60f511fb83547a5d62653252;p=ghc-hetmet.git

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 }