X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FhsSyn%2FHsPat.lhs;h=25a350b43a9df9d24734c3757fb6830624015949;hb=9ba922ee06b048774d7a82964867ff768a78126e;hp=1bf0aa828148bd7cf020f820e883c7a8bf2465e7;hpb=d2ce0f52d42edf32bb9f13796e6ba6edba8bd516;p=ghc-hetmet.git

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