Typed template haskell quote fails to typecheck when spliced due to an ambiguous type variable

It should be the case that a code value constructed using typed template haskell should never fail to type check when spliced. Running ghc Test.hs with the following two modules produces an error about an ambiguous type variable.

https://gist.github.com/5890c14dda73da738d2041c7f677b786

{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -Wall #-}
module Compiler where

import Language.Haskell.TH

data Operator = Scan
              | Join Operator Operator deriving Show

queryJoin :: Operator
queryJoin = Join Scan Scan

type QTExp a = Q (TExp a)

fix :: (a -> a) -> a
fix f = let x = f x in x

while ::
  Monoid m =>
  QTExp (IO m -> IO m) -> QTExp (IO m)
while b = [|| fix (\r -> whenM True ($$b r)) ||]

whenM :: Monoid m => Bool -> m -> m
whenM b act = if b then act else mempty

execOp :: Monoid m => Operator -> QTExp (IO m) -> QTExp (IO m)
execOp op yld =
  case op of
    Scan ->
      while [|| \r -> ($$(yld) >> r)||]
    Join left right ->
      execOp left (execOp right yld)

runQuery :: QTExp (IO ())
runQuery = execOp (Join Scan Scan) ([|| return () ||])

{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Test where

import qualified Compiler as C

main :: IO ()
main = do
$$(C.runQuery)

Test.hs:9:6: error:
    • Ambiguous type variable ‘a0’ arising from a use of ‘C.whenM’
      prevents the constraint ‘(Monoid a0)’ from being solved.
      Relevant bindings include r_a5GX :: IO a0 (bound at Test.hs:9:6)
      Probable fix: use a type annotation to specify what ‘a0’ should be.
      These potential instances exist:
        instance Monoid a => Monoid (IO a) -- Defined in ‘GHC.Base’
        instance Monoid Ordering -- Defined in ‘GHC.Base’
        instance Semigroup a => Monoid (Maybe a) -- Defined in ‘GHC.Base’
        ...plus 7 others
        (use -fprint-potential-instances to see them all)
    • In the expression:
        (C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX)
      In the first argument of ‘C.fix’, namely
        ‘(\ r_a5GX
            -> (C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX))’
      In the first argument of ‘(>>)’, namely
        ‘(C.fix
            (\ r_a5GX
               -> (C.whenM True) ((\ r_a5GY -> ((return ()) >> r_a5GY)) r_a5GX)))’
  |
9 |   $$(C.runQuery)
  |  

The generated code

Test.hs:9:6-15: Splicing expression
    C.runQuery
  ======>
    C.fix
      (\ r_a5GV
         -> (C.whenM True)
              ((\ r_a5GW
                  -> ((C.fix
                         (\ r_a5GX
                            -> (C.whenM True)
                                 ((\ r_a5GY -> ((return GHC.Tuple.()) >> r_a5GY)) r_a5GX)))
                        >> r_a5GW))
                 r_a5GV))

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Operating system
Architecture