From b09cd42dbcead91f06e01119a1365af88ce8de7c Mon Sep 17 00:00:00 2001
From: Michael Snoyman <michael@snoyman.com>
Date: Wed, 24 Jun 2015 04:40:28 +0300
Subject: [PATCH] Fix incorrect assertion on Windows build plan shadowing

---
 src/Stack/BuildPlan.hs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/Stack/BuildPlan.hs b/src/Stack/BuildPlan.hs
index 6cd85e4bb7..0c8bcabfb4 100644
--- a/src/Stack/BuildPlan.hs
+++ b/src/Stack/BuildPlan.hs
@@ -594,7 +594,15 @@ shadowMiniBuildPlan (MiniBuildPlan ghc pkgs0) shadowed =
                 Just x -> return x
                 Nothing ->
                     case Map.lookup name pkgs1 of
-                        Nothing -> assert (name `Set.member` shadowed) (return False)
+                        Nothing
+                            | name `Set.member` shadowed -> return False
+
+                            -- In this case, we have to assume that we're
+                            -- constructing a build plan on a different OS or
+                            -- architecture, and therefore different packages
+                            -- are being chosen. The common example of this is
+                            -- the Win32 package.
+                            | otherwise -> return True
                         Just mpi -> do
                             let visited' = Set.insert name visited
                             ress <- mapM (check visited') (Set.toList $ mpiPackageDeps mpi)