Make isIrrefutableHsPat say True for existentials; fixes Trac #4439

[ghc-hetmet.git] / compiler / hsSyn / HsPat.lhs

diff --git a/compiler/hsSyn/HsPat.lhs b/compiler/hsSyn/HsPat.lhs
index 1bf0aa8..25a350b 100644 (file)
--- 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