From cbe382d4ff5a37d38a56aa346b9d229d336f16b4 Mon Sep 17 00:00:00 2001
From: "simonpj@microsoft.com" <unknown>
Date: Tue, 26 Oct 2010 07:04:05 +0000
Subject: [PATCH] Make isIrrefutableHsPat say True for existentials; fixes
 Trac #4439

At last. A bug that is really easy to fix.
---
 compiler/hsSyn/HsPat.lhs |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/compiler/hsSyn/HsPat.lhs b/compiler/hsSyn/HsPat.lhs
index 1bf0aa8..25a350b 100644
--- a/compiler/hsSyn/HsPat.lhs
+++ b/compiler/hsSyn/HsPat.lhs
@@ -47,6 +47,7 @@ import SrcLoc
 import FastString
 -- libraries:
 import Data.Data hiding (TyCon)
+import Data.Maybe
 \end{code}
 
 
@@ -411,7 +412,9 @@ isIrrefutableHsPat pat
 
     go1 (ConPatIn {})       = False	-- Conservative
     go1 (ConPatOut{ pat_con = L _ con, pat_args = details }) 
-	=  isProductTyCon (dataConTyCon con)
+	=  isJust (tyConSingleDataCon_maybe (dataConTyCon con))
+	   -- NB: tyConSingleDataCon_maybe, *not* isProductTyCon, because 
+	   -- the latter is false of existentials. See Trac #4439
 	&& all go (hsConPatArgs details)
 
     go1 (LitPat {})    = False
-- 
1.7.10.4