From 6344b1506042f6e150c1b105c6696f3d75a5eb66 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Fri, 1 Aug 2008 12:22:23 +0000
Subject: [PATCH] Fix Trac #2478

A minor glitch that shows up only when a data constructor has *both* a
"stupid theta" in the data type decl, *and* an existential type variable.
---
 compiler/typecheck/TcPat.lhs |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/compiler/typecheck/TcPat.lhs b/compiler/typecheck/TcPat.lhs
index 6a30754..2e86583 100644
--- a/compiler/typecheck/TcPat.lhs
+++ b/compiler/typecheck/TcPat.lhs
@@ -799,7 +799,9 @@ addDataConStupidTheta data_con inst_tys
 	-- The origin should always report "occurrence of C"
 	-- even when C occurs in a pattern
     stupid_theta = dataConStupidTheta data_con
-    tenv = zipTopTvSubst (dataConUnivTyVars data_con) inst_tys
+    tenv = mkTopTvSubst (dataConUnivTyVars data_con `zip` inst_tys)
+    	 -- NB: inst_tys can be longer than the univ tyvars
+	 --     because the constructor might have existentials
     inst_theta = substTheta tenv stupid_theta
 \end{code}
 
-- 
1.7.10.4