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
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug TemplateHaskell Trac import labels

Some quick checking indicates the error message is using invalid syntax too: $[t| X |] is not accepted by GHC.

mentioned in commit 127d259f

I've fixed the pretty-printer so that it doesn't print $[t| X |].

mentioned in issue #4230 (closed)

This too is related to #4230 (closed).

Trac metadata

Trac field	Value
BlockedBy	- → #4230 (closed)

changed milestone to %7.0.2

changed milestone to %7.2.1

changed milestone to %7.4.1

changed milestone to %7.6.1

changed weight to 3

Trac metadata

Trac field	Value
Priority	normal → low

changed milestone to %7.6.2

mentioned in commit aa8559d7

closed

This is now fixed thanks to new TH.

BTW for older GHC you could work around it by enabling -XFlexibleInstances (as the error suggests), but that was unneccessary and is now not needed.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed
Test case	→ th/4124
BlockedBy	#4230 (closed) →

added Plow label