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, 10 insertions, 0 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
index fb79a9d973..539e2220b7 100644
--- a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
@@ -38,6 +38,10 @@ import Control.Monad.Trans.Writer
 #else
 import Data.Monoid ( Monoid, mappend, mempty )
 #endif
+#if __GLASGOW_HASKELL__ > 710
+import Data.Semigroup   ( Semigroup )
+import qualified Data.Semigroup as Semigroup
+#endif
 import Data.List ( nub )
 import Data.Maybe ( catMaybes )
 
@@ -1840,6 +1844,12 @@ getTBAARegMeta = getTBAAMeta . getTBAA
 -- | A more convenient way of accumulating LLVM statements and declarations.
 data LlvmAccum = LlvmAccum LlvmStatements [LlvmCmmDecl]
 
+#if __GLASGOW_HASKELL__ > 710
+instance Semigroup LlvmAccum where
+  LlvmAccum stmtsA declsA <> LlvmAccum stmtsB declsB =
+        LlvmAccum (stmtsA Semigroup.<> stmtsB) (declsA Semigroup.<> declsB)
+#endif
+
 instance Monoid LlvmAccum where
     mempty = LlvmAccum nilOL []
     LlvmAccum stmtsA declsA `mappend` LlvmAccum stmtsB declsB =