Add subsumption example to GHC-83865

message-index/messages/GHC-83865/subsumption/after/Subsumption.hs

@@ -0,0 +1,5 @@
+f :: (forall r. r -> r) -> Int
+f x = g x
+
+g :: (String -> String) -> Int
+g _ = 1337

message-index/messages/GHC-83865/subsumption/before/Subsumption.hs

@@ -0,0 +1,5 @@
+f :: (forall r. r -> r) -> Int
+f = g
+
+g :: (String -> String) -> Int
+g _ = 1337

message-index/messages/GHC-83865/subsumption/index.md

@@ -0,0 +1,36 @@
+---
+title: Forall-quantification not matched
+order: 2
+---
+
+Function `f` takes an argument of type `forall r. r -> r`, so you might think
+the function `g :: String -> String` would be suitable, because the type variable 
+`r` can be instantiated to the concrete type `String`.
+
+However, GHC 9.0 and later do not instantiate of forall-quantified variables in function arguments like that.
+
+You can almost always fix this issue by explicitly applying arguments as shown in the "after" column below.
+In fact, that is what GHC used to do automatically.
+Note that this can prevent sharing in some situation, which is why it was deemed better to make this explicit.
+
+Since GHC 9.2.4, you can also enable the `DeepSubsumption` language extension to fix this error which reverts GHC back to its old implicit behavior.
+
+For more detailed information see:
+
+* Youtube: [@rae: What Haskell's deep subsumption is, why we killed it, and then why we resurrected it.](https://www.youtube.com/watch?v=XMnXbBRg-B0)
+* GHC Proposal: [Simplify subsumption](https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst)
+
+## Error Message
+
+```
+Subsumption.hs:2:5: error: [GHC-83865]
+    • Couldn't match type: String -> String
+                     with: forall r. r -> r
+      Expected: (forall r. r -> r) -> Int
+        Actual: (String -> String) -> Int
+    • In the expression: g
+      In an equation for ‘f’: f = g
+  |
+2 | f = g
+  |     ^
+```