Promotable type synonyms (or: synonyms to promoted types)

I want to use synonyms to promoted types in the same places where I can use the promoted types:

{-# LANGUAGE DataKinds, KindSignatures #-}
module PromotableTypeSynonyms where

data B = T | F
data P :: B -> *

type B' = B
data P' :: B' -> * -- using B' synonym here is currently not allowed

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Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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