testsuite: Add a test for #18923

testsuite/tests/perf/compiler/T18923.hs

+module T18923 (mergeRec, Rec) where
+
+mayMerge :: Maybe b -> Maybe b -> Maybe b
+mayMerge Nothing  y        = y
+mayMerge x        Nothing  = x
+mayMerge (Just x) (Just y) = Just y
+
+data Rec = Rec { v0,v1,v2,v3,v4,v5,v6,v7 :: !(Maybe Bool) }
+
+mergeRec :: Rec -> Rec -> Rec
+mergeRec
+  (Rec a0 a1 a2 a3 a4 a5 a6 a7)
+  (Rec b0 b1 b2 b3 b4 b5 b6 b7) =
+    Rec (mayMerge a0 b0) (mayMerge a1 b1) (mayMerge a2 b2) (mayMerge a3 b3)
+        (mayMerge a4 b4) (mayMerge a5 b5) (mayMerge a6 b6) (mayMerge a7 b7)
+

testsuite/tests/perf/compiler/all.T

@@ -434,3 +434,7 @@ test ('T18223',
       ],
       compile,
       ['-v0 -O'])
+test ('T18923',
+      [ collect_compiler_stats('bytes allocated',2) ],
+      compile,
+      ['-v0 -O'])