migration/8.6.md

@@ -7,6 +7,30 @@ This guide summarises the changes you may need to make to your code to migrate f
 
 ## Compiler changes
 
+### `PolyKinds` and `TypeInType` are pickier
+
+
+GHC is now more diligent about catching illegal uses of kind polymorphism that snuck into recent GHC releases. For instance, this used to be accepted without `PolyKinds`:
+
+```
+classC a where
+  c ::Proxy(x :: a)
+```
+
+
+Despite the fact that `a` is used as a kind variable in the type signature for `c`. This is now an error unless `PolyKinds` is explicitly enabled.
+
+
+Moreover, GHC 8.4 would accept the following without the use of `TypeInType` (or even `PolyKinds`!):
+
+```
+f:: forall k (a :: k).Proxy a
+f=Proxy
+```
+
+
+Despite the fact that `k` is used as both a type and kind variable. This is now an error unless `TypeInType` is explicitly enabled.
+
 ---
 
 ## Library changes