Illegal standalone kind signature [GHC-45906]

Standalone kind signatures can be used to specify a polymorphic kind for a type, to overcome kind inference restrictions:

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.

Examples

Illegal standalone kind signature

Module fails to compile because it contains a standalone type signature.

Error Message

SAKS.hs:7:1: error: [GHC-45906]
    Illegal standalone kind signature
    Suggested fix:
      Perhaps you intended to use the ‘StandaloneKindSignatures’ extension (implied by ‘UnliftedDatatypes’)

SAKS.hs

Before

{-# LANGUAGE Haskell2010 #-}

module SAKS where

import Data.Kind (Type)

type T :: Type
data T

After

{-# LANGUAGE GHC2021 #-}

module SAKS where

import Data.Kind (Type)

type T :: Type
data T