{-# LANGUAGE TemplateHaskell #-}
module Fixity where

class MyClass a where
   (.*.) :: a -> a -> a

main = putStrLn "hi"

-- $( [d| x = undefined |] )

infixr 3 .*.

Compiling the above with: c:\ghc\ghc-6.10.0.20081007\bin\ghc.exe --make -ddump-hi ~/Fixity.hs gives:

==================== FINAL INTERFACE ====================
interface main:Fixity 610020081007
 interface hash: 791ca25a8ddea4dee47d108320bf5b50
 ABI hash: c2ca52d8c1ffc89265effb15d766f4aa
 export-list hash: 9613c0af035884a108b9bebc1c0b08b6
 orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e
 where
export main:Fixity MyClass|{.*. MyClass} main
module dependencies:
package dependencies: ghc-prim integer base
orphans: base:Control.Exception.Base base:Data.Tuple base:GHC.Base
        base:GHC.Float base:GHC.Num
family instance modules:
import base:Prelude eac9ca3c0ae6d00a0fc0f1832a247e5e
import base:System.IO 9fa582151667de94fbfb9074e3c0f1b4
fixities infixr 3 .*.
1bcd9343d420eae1078ef2fa9021dbf8
 class MyClass a
     RecFlag NonRecursive
     .*. :: a -> a -> a
96fb86d3649acbcf89c1f2802a511b0e
 main :: GHC.IOBase.IO ()
vectorised variables:
vectorised tycons:
vectorised reused tycons:

Which is fine. Uncommenting the TH line and recompiling though gives:

==================== FINAL INTERFACE ====================
interface main:Fixity 610020081007
 interface hash: 0a7949a029127113f8c49434e22bcb71
 ABI hash: 3e2e66371e835a766a2fdedd54cb2c35
 export-list hash: 73f4a7845d4b5048d9321b0e99b46428
 orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e
 where
export main:Fixity MyClass|{.*. MyClass} main x
module dependencies:
package dependencies: ghc-prim integer base template-haskell
orphans: base:Control.Exception.Base base:Data.Tuple base:GHC.Base
        base:GHC.Float base:GHC.Num
family instance modules:
import base:GHC.Err 357889242e9fa603050a70e6dac2a5e4
import base:Prelude eac9ca3c0ae6d00a0fc0f1832a247e5e
import base:System.IO 9fa582151667de94fbfb9074e3c0f1b4
852943708ad257bdcf76e94914bb84b2
 class MyClass a
     RecFlag NonRecursive
     .*. :: a -> a -> a
96fb86d3649acbcf89c1f2802a511b0e
 main :: GHC.IOBase.IO ()
41850025600a6740465944638d138bc4
 x :: forall a. a
vectorised variables:
vectorised tycons:
vectorised reused tycons:

1. ..ie the "fixities" entry is lost.

This worked fine in 6.8.2.

http://hackage.haskell.org/trac/ghc/ticket/2632 may be related - but that was marked as closed/merged on 4th Oct, and I'm using a GHC