When I say

{-# LANGUAGE TemplateHaskell, TypeFamilies #-}
{-# OPTIONS_GHC -ddump-splices -dsuppress-uniques #-}

module Bug where

$([d| class C a where
        type F a
        type F a = a |])

I get

    [d| class C a where
          type family F a
          F a = a |]
  ======>

Bug.hs:6:7: Warning:
    Cannot desugar this Template Haskell declaration:
        class C a where
          type family F a
          F a = a

There are two problems here:

1. This really should work.
2. The pretty-printer here omits the type on the default definition for F. (The word family is optional there, but isn't actually wrong.)