Fix build with ghc 8.4+, which broke due to the Semigroup Monoid change

https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid

I am not happy with the fragile pile of CPP boilerplate required to support
ghc back to 7.0, which git-annex still targets for both the android build
and the standalone build targeting old linux kernels. It makes me unlikely
to want to use Semigroup more in git-annex, because the benefit of the
abstraction is swamped by the ugliness. I actually considered ripping out
all the Semigroup instances, but some are needed to use
optparse-applicative.

The problem, I think, is they made this transaction on too fast a timeline.
(Although ironically, work on it started in 2015 or earlier!)
In particular, Debian oldstable is not out of security support, and it's
not possible to follow the simpler workarounds documented on the wiki and
have it build on oldstable (because the semigroups package in it is too
old).

I have only tested this build with ghc 8.2.2, not the newer and older
versions that branches of the CPP support. So there could be typoes, we'll
see.

This commit was sponsored by Brock Spratlen on Patreon.

1 files changed, 24 insertions, 6 deletions

diff --git a/Types/Test.hs b/Types/Test.hs
index 50c460f501..5a9a9e075c 100644
--- a/Types/Test.hs
+++ b/Types/Test.hs
@@ -5,11 +5,17 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Types.Test where
 
 import Test.Tasty.Options
 import Data.Monoid
+#if MIN_VERSION_base(4,9,0)
+import qualified Data.Semigroup as Sem
+#endif
 import Prelude
+
 import Types.Command
 
 data TestOptions = TestOptions
@@ -19,13 +25,25 @@ data TestOptions = TestOptions
 	, internalData :: CmdParams
 	}
 
+appendTestOptions :: TestOptions -> TestOptions -> TestOptions
+appendTestOptions a b = TestOptions
+	(tastyOptionSet a <> tastyOptionSet b)
+	(keepFailuresOption a || keepFailuresOption b)
+	(fakeSsh a || fakeSsh b)
+	(internalData a <> internalData b)
+
+#if MIN_VERSION_base(4,9,0)
+instance Sem.Semigroup TestOptions where
+	(<>) = appendTestOptions
+#endif
+
 instance Monoid TestOptions where
 	mempty = TestOptions mempty False False mempty
-	mappend a b = TestOptions
-		(tastyOptionSet a <> tastyOptionSet b)
-		(keepFailuresOption a || keepFailuresOption b)
-		(fakeSsh a || fakeSsh b)
-		(internalData a <> internalData b)
-
+#if MIN_VERSION_base(4,11,0)
+#elif MIN_VERSION_base(4,9,0)
+	mappend = (Sem.<>)
+#else
+	mappend = appendTestOptions
+#endif
 
 type TestRunner = TestOptions -> IO ()