Allow fusion with catMaybes (#18574)

Metric Decrease:
   T18574

libraries/base/Data/Maybe.hs

@@ -256,8 +256,8 @@ listToMaybe = foldr (const . Just) Nothing
 -- >>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
 -- [1,3]
 --
-catMaybes              :: [Maybe a] -> [a]
-catMaybes ls = [x | Just x <- ls]
+catMaybes :: [Maybe a] -> [a]
+catMaybes = mapMaybe id -- use mapMaybe to allow fusion (#18574)
 
 -- | The 'mapMaybe' function is a version of 'map' which can throw
 -- out elements.  In particular, the functional argument returns

libraries/base/changelog.md

@@ -32,6 +32,9 @@
   * Add `MonadFix` and `MonadZip` instances for `Complex`
 
   * Add `Ix` instances for tuples of size 6 through 15
+
+  * `catMaybes` is now implemented using `mapMaybe`, so that it is both a "good
+    consumer" and "good producer" for list-fusion (#18574)
    
 ## 4.14.0.0 *TBA*
   * Bundled with GHC 8.10.1