Implement warnings for Semigroups as parent of Monoid

This patch is similar to the AMP patch (#8004), which offered two
functions:

  1. Warn when an instance of a class has been given, but the type does
     not have a certain superclass instance
  2. Warn when top-level definitions conflict with future Prelude names

These warnings are issued as part of the new `-Wcompat` warning group.

Reviewers: hvr, ekmett, austin, bgamari

Reviewed By: hvr, ekmett, bgamari

Subscribers: ekmett, thomie

Differential Revision: https://phabricator.haskell.org/D1539

GHC Trac Issues: #11139

1 files changed, 16 insertions, 0 deletions

diff --git a/compiler/main/Packages.hs b/compiler/main/Packages.hs
index fdf96708fb..ac4fae2449 100644
--- a/compiler/main/Packages.hs
+++ b/compiler/main/Packages.hs
@@ -79,6 +79,10 @@ import Data.Set (Set)
 #if __GLASGOW_HASKELL__ < 709
 import Data.Monoid hiding ((<>))
 #endif
+#if __GLASGOW_HASKELL__ > 710
+import Data.Semigroup   ( Semigroup )
+import qualified Data.Semigroup as Semigroup
+#endif
 import qualified Data.Map as Map
 import qualified FiniteMap as Map
 import qualified Data.Set as Set
@@ -191,6 +195,18 @@ fromReexportedModules False pkg = ModOrigin Nothing [] [pkg] False
 fromFlag :: ModuleOrigin
 fromFlag = ModOrigin Nothing [] [] True
 
+#if __GLASGOW_HASKELL__ > 710
+instance Semigroup ModuleOrigin where
+    ModOrigin e res rhs f <> ModOrigin e' res' rhs' f' =
+        ModOrigin (g e e') (res ++ res') (rhs ++ rhs') (f || f')
+      where g (Just b) (Just b')
+                | b == b'   = Just b
+                | otherwise = panic "ModOrigin: package both exposed/hidden"
+            g Nothing x = x
+            g x Nothing = x
+    _x <> _y = panic "ModOrigin: hidden module redefined"
+#endif
+
 instance Monoid ModuleOrigin where
     mempty = ModOrigin Nothing [] [] False
     mappend (ModOrigin e res rhs f) (ModOrigin e' res' rhs' f') =