template haskell: incorrect Warning: defined but not used

example code:

{-# LANGUAGE TemplateHaskell #-}

module THWarnBug where

import Language.Haskell.TH

mhlt :: Q Type
mhlt = do
  let u = [t| String |]
  [t| $u |]

compiling gives:

$ ghc -c -Wall THWarnBug.hs 

THWarnBug.hs:9:6: Warning: Defined but not used: `u'

but obviously u is used.

Trac metadata

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