This bit me just now, so I figured I should report a feature request bug.

ezyang@sabre:~/Dev/whisk$ ghc-8.0 --interactive -XTemplateHaskell
GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/  :? for help
Prelude> Language.Haskell.TH.runQ [| \(x :: ()) -> x |]

<interactive>:1:29: error:
    Type signatures in patterns not (yet) handled by Template Haskell
      x :: ()

In the code, there is a bit of mumbling about scoped type variables. I believe it is referring to code like this:

Prelude> :t (\(x :: a) -> (x :: a))
(\(x :: a) -> (x :: a)) :: a -> a

Described in the manual here: https://downloads.haskell.org/\~ghc/latest/docs/html/users_guide/glasgow_exts.html\#pattern-type-signatures This means that TH has to somehow bring the type into scope. I don't actually know how to do it.

It seems a bit awful for this to be the reason why these quotes are not accepted; maybe without scoped type variables we should be able to do something cheap and cheerful.