deprecate NFData on function types for base-4.20 and up

Author: mixphix

Control/DeepSeq.hs

@@ -531,7 +531,11 @@ instance NFData1 Fixed where liftRnf _ = rwhnf
 --  This assumes that WHNF is equivalent to NF for functions.
 --
 --  @since 1.3.0.0
+#if MIN_VERSION_base(4,20,0)
+instance {-# DEPRECATED "NFData is not well-defined on function types." #-} NFData (a -> b) where rnf = rwhnf
+#else
 instance NFData (a -> b) where rnf = rwhnf
+#endif
 
 -- Rational and complex numbers.
 

changelog.md

@@ -2,7 +2,8 @@
 
 ## Upcoming
 
-
+  * Deprecate function instance starting in GHC 9.10 (`base-4.20`)
+    ([#16](https://github.com/haskell/deepseq/issues/16), [#47](https://github.com/haskell/deepseq/pull/47))
 
 ## 1.5.1.0