[commit: ghc] type-nats: Add some extra impossible cases (x + 5 ~ x) (0d898ce)

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : type-nats

http://hackage.haskell.org/trac/ghc/changeset/0d898cef94e7693a876a013ff4bb50dac4a0f943

>---------------------------------------------------------------

commit 0d898cef94e7693a876a013ff4bb50dac4a0f943
Author: Iavor S. Diatchki <[email protected]>
Date:   Sun Sep 16 15:22:36 2012 -0700

    Add some extra impossible cases (x + 5 ~ x)

>---------------------------------------------------------------

 compiler/typecheck/TcTypeNats.hs |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/compiler/typecheck/TcTypeNats.hs b/compiler/typecheck/TcTypeNats.hs
index d4531c2..9bfcfea 100644
--- a/compiler/typecheck/TcTypeNats.hs
+++ b/compiler/typecheck/TcTypeNats.hs
@@ -174,6 +174,8 @@ impossible (CFunEqCan { cc_fun = tc, cc_tyargs = [t1,t2], cc_rhs = t3 })
       case (mbA,mbB,mbC) of
         (Just a, _     , Just c) -> isNothing (minus c a)
         (_     , Just b, Just c) -> isNothing (minus c b)
+        (Just a, _     , _) | a > 0 -> eqType t2 t3
+        (_     , Just b, _) | b > 0 -> eqType t1 t3
         _                        -> False
 
   | name == typeNatMulTyFamName =