Add GHC-45906 (#508)

Author: frasertweedale

message-index/messages/GHC-45906/example/after/SAKS.hs

@@ -0,0 +1,8 @@
+{-# LANGUAGE GHC2021 #-}
+
+module SAKS where
+
+import Data.Kind (Type)
+
+type T :: Type
+data T

message-index/messages/GHC-45906/example/before/SAKS.hs

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+{-# LANGUAGE Haskell2010 #-}
+
+module SAKS where
+
+import Data.Kind (Type)
+
+type T :: Type
+data T

message-index/messages/GHC-45906/example/index.md

@@ -0,0 +1,15 @@
+---
+title: Illegal standalone kind signature
+---
+
+Module fails to compile because it contains a standalone type
+signature.
+
+## Error Message
+
+```haskell
+SAKS.hs:7:1: error: [GHC-45906]
+    Illegal standalone kind signature
+    Suggested fix:
+      Perhaps you intended to use the ‘StandaloneKindSignatures’ extension (implied by ‘UnliftedDatatypes’)
+```

message-index/messages/GHC-45906/index.md

@@ -0,0 +1,24 @@
+---
+title: Illegal standalone kind signature
+summary: Program contains a standalone kind signature, but the active extensions do not allow it.
+severity: error
+introduced: 9.6.1
+---
+
+[Standalone kind signatures] can be used to specify a polymorphic
+kind for a type, to overcome kind inference restrictions:
+
+```haskell
+type T :: (k -> Type) -> k -> Type  -- standalone kind signature
+data T m a = MkT (m a) (T Maybe (m a))
+```
+
+Without the standalone kind signature, GHC would infer the kind
+`T :: (Type -> Type) -> Type -> Type`.
+
+Use of this feature requires the `StandaloneKindSignatures`
+extension to be enabled.  This extension is implied by
+`UnliftedDatatypes`, and included in the `GHC2021` and `GHC2024`
+language editions.
+
+[Standalone kind signatures]: https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/exts/poly_kinds.html#standalone-kind-signatures-and-polymorphic-recursion