GHC rejects instance quotations with splices in the instance head

I think the following code should be accepted.

{-# LANGUAGE TemplateHaskell #-}
class Storable a where
data X = X
[d| instance Storable $( [t| X |] ) where |]

GHC disagrees, saying:

test.hs:4:4:
    Illegal instance declaration for `Storable t_aKj'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are type *variables*,
         and each type variable appears at most once in the instance head.
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `Storable $[t| X |]'
    In the Template Haskell quotation
      [d| 
          instance Storable $[t| X |] where |]
    In the expression:
        [d| 
            instance Storable $[t| X |] where |]

This checking seems inappropriate before the type is actually spliced in!

Trac metadata

Trac field	Value
Version	6.12.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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