From 0e21678677894ed7cb236499ec6f2041efae22a2 Mon Sep 17 00:00:00 2001
From: Herbert Valerio Riedel <hvr@gnu.org>
Date: Tue, 3 Nov 2015 16:18:30 -0600
Subject: [PATCH] Cabal-level sanity check to enforce Cabal flag-invariant

This helps catch cabal-flag misconfiguration early on during `cabal
configure`, rather than much later in the build-process during
compilation.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1423
---
 libraries/base/base.cabal | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/libraries/base/base.cabal b/libraries/base/base.cabal
index d98e7bfc00..e94d38949a 100644
--- a/libraries/base/base.cabal
+++ b/libraries/base/base.cabal
@@ -94,6 +94,11 @@ Library
         Unsafe
 
     build-depends: rts == 1.0.*, ghc-prim == 0.5.*
+
+    -- sanity-check to ensure exactly one flag is set
+    if !((flag(integer-gmp) && !flag(integer-simple)) || (!flag(integer-gmp) && flag(integer-simple)))
+        build-depends: invalid-cabal-flag-settings<0
+
     if flag(integer-simple)
         build-depends: integer-simple >= 0.1.1 && < 0.2
 
-- 
GitLab